UPDATE 9:30 AM, 5/13: Some scattered frost reported in higher elevations and locations west of Roanoke this morning with lows in the 30s common — official low of 40 at Roanoke. A new freeze watch is already posted north of Roanoke for Tuesday morning, with more widespread frost advisories likely to be issued later. A few sprinkles are possible today with a quick-passing disturbance, but no major rain expected. This will be a week of winter-to-summer extremes, with some 80s temperatures later in the week. Full update this evening. END UPDATE

HAPPY MOTHER’S DAY! Expect a sunny day with temperatures rising into the 60s most places in Southwest Virginia. It will be a bit breezy and cool, but dry.

As on Friday, a smattering of severe weather reports occurred Saturday in Virginia (including trees blown down near Thaxton in Bedford County) with storms that at time looked quite impressive at times but were marginal on shear, instability and available moisture. My storm chasing friend and Roanoke resident Chris White of Roanoke photographed some formations with obvious rotation in central Virginia. I stayed closer to home and got a shot of a lowering with a rain shaft (sortakinda looks like a tornado, but it’s not) crossing the Roanoke Valley (was actually with my mother when I shot this, appropriate for the weekend, I suppose).  Here’s a nice approaching storm shot from the Valley View area tweeted to me by @Hokie_94.  Storm chasing and photography will be very much in our discussions the next few weeks — the first Virginia Tech storm chase trip leaves Blacksburg Tuesday (with Chris White as a co-leader) and the second leaves around Memorial Day (with me as a co-leader)! I think the first-trip group will get some central U.S. severe weather action next weekend.

The cold front that caused the storms has pushed through and a cooler, drier air mass is settling in for the next 3 days or so. We’ve talked a few days about the potential for freezing temperatures and/or frost both Monday morning and Tuesday morning in at least part of our region. The National Weather Service in Blacksburg has issued a freeze watch for Monday morning some counties along the West Virginia border (Craig, Giles, Bland) and along and west of the I-77 corridor (Tazewell, Smyth, Grayson) as well as mountainous counties in its forecast area in West Virginia and North Carolina. I would not be surprised to see freeze watches or warnings eventually issued a little farther east — especially on Tuesday morning — and frost advisories expanded to include most of our region, even into Southside and Piedmont counties, over the next couple of days. Temperatures near or below freezing are likely in most of the area west of Roanoke on Monday and Tuesday mornings, plus some outlying areas and sheltered valleys elsewhere. On Tuesday, the May 14 record lows of 31 at Blacksburg and 37 at Roanoke (both set in 1996) could be in jeopardy. Maximum radiational cooling conditions — clear skies and calm winds for several hours — could also challenge Roanoke’s latest freeze date, which, dating to 1912, was May 11, 1966. Short summary: It would be wide to protect tender outdoor vegetation on Monday and Tuesday mornings, even in the urban areas of Roanoke city.

We’ll shoot back into the 80s by Thursday. As we challenge May records for cold on Tuesday, temperatures may push toward 90 in what has been a very cold Upper Midwest this spring, places like Chicago and Minneapolis. The topsy-turvy, sometimes upside-down spring continues.

My thanks to Zach Robinson for taking care of the blog content for the past 3 days while I was on a short beach vacation with my family in South Carolina. We had one day of heavy rain but a good amount of sunshine the other two.

In case you missed it, Wednesday’s Weather Journal column (linked here) in The Roanoke Times looked back at the late April 1978 heavy rain/snow storm in Southwest Virginia. Seems kind of fitting now in light of what is happening to our west.

The central U.S. is experiencing a historic weather event as upper-level energy will cut off a low-pressure system aloft and its pocket of extraordinarily cold air for the season (Zach covered this well the last couple of days). At left is the Weather Prediction Center’s map for chances of at least 1 inch of snow through Friday morning – it extends a lot farther north, and even up toward Iowa and Minnesota, some of the amounts could be quite unusual for May, even historically large (in some cases, potentially breaking snowfall records for the entire month of May). On the south end, though, it borders on the ridiculous to even consider snow. The state of Arkansas, for instance, has no official records of snow having ever fallen in May dating back to the 1800s,  even a trace, yet has a coin’s-flip chance (blue colors) of  seeing an inch of wet snow edging into the northwest counties. And some parts of eastern Kansas with elevations lower than those along the banks of the Roanoke River have a 70 percent or greater chance (light blue, pink) of seeing an inch. There may also be some May monthly low temperature records set the next couple of days, especially in the southern Plains/lower Mississippi River Valley.

How will this affect Southwest Virginia? In the short term, the cutoff low in the central U.S. will be part of an “omega block” pattern — low to our west, low offshore, high pressure trapped in the middle — that will keep seasonable to slightly below normal daytime temperatures (60s to low 70s) and mostly dry conditions in place, aside from some drizzle or fog as Atlantic moisture is lifted against our higher terrain on easterly to northeasterly winds. But in time, it is likely the cutoff low will wobble eastward toward us. The cold air will modify quite a bit, so we won’t experience the extraordinary level of cold the central U.S. is, but its counterclockwise circulation is likely to rotate thick moisture our way from the Gulf of Mexico and Atlantic Ocean early next week. We may experience quite a rainy period from Sunday to Tuesday or so early next week. Spring just isn’t taking any hints yet to break out sunny and warm for any extended periods of time.

I will be taking Sunday through Wednesday off from Weather Journal. Zach Robinson will be filling in for me with blog entries on Monday, Tuesday and Wednesday. Zach is a frequent Weather Journal commenter from Fancy Gap who blogs about weather on his own Blue Ridge Weather Blog and was noted in this space in early March for being ahead of the curve on shifting the bulk of the early March snowstorm generally north of our area. I’ll have more on Zach in my Saturday night/Sunday morning post.

The overriding theme for the next 7-10 days in Southwest Virginia, possibly longer, is simple: Cool and wet. The Climate Prediction Center’s 6-10 day and 8-14-day maps clearly reflect this trend.  It’s really been the overriding theme, minus a few short windows, since mid January. Saturday will be a pretty good day for outdoor activities, with highs in the 60s — clouds will increase, but rain likely holds off til well after dark. Sunday, though, will be a raw, chilly, damp day with highs likely not making 60, when mid 60s to low 70s are normal. The combination of high pressure to the northeast banking cool air against the Appalachians and moisture spread into and over that cool air by an approaching low-pressure system from the southwest will be like a damp rag lying over us all day. Whatever the temperature is when you get up Sunday morning (upper 40s to low 50s, perhaps)  it will wiggle upward very begrudgingly, if at all, as rain, low clouds, fog and drizzle dominate the day. Rainfall amounts through Monday could be near an inch in our region, with much heavier rain to the south.  When the low finally moves offshore by Tuesday, we’ll see more sunshine, and some highs perhaps near normal mid 60s to low 70s for midweek, with the high to the northeast and a weak wedge deterring any runaway warming.

It is worth noting now that about a week from now, a strong Canadian cold front is poised to move across the central and eastern U.S. Some forecast models develop a cutoff low-pressure system that would cause unseasonably cold, damp weather to become stuck somewhere over the central or eastern U.S. for a few days. with the central/southern Appalachians currently favored by the consensus of models and forecasters. There is even the possibility that some snow could fall very far to the south for early May, especially at higher elevations. The departure of the cold front late next week may also lead to freezing temperatures dipping into parts of the South that are normally free of such threats this time of year. It’s all a long time away — a good time to step away and let Zach track how it may unfold for a few days.

Wednesday’s Weather Journal column looks at how tornado numbers have been low nationally for about the last year. The reasons have differed dramatically during the typical spring peak seasons — unseasonable warmth in 2012, and unseasonable chill in 2013. Some related reading to the latter is provided by the Washington Post’s Capital Weather Gang, which looks at the extraordinarily cold and snowy April in the north-central U.S. — including the interesting fact that Duluth, Minn., had more snow in April (51 inches) than any previous month on record — not any previous April, any previous month.

A surge of warmth is headed to Southwest Virginia on Wednesday that may carry us to temperatures in the mid 70s to low 80s, about 10 degrees above normal. But it will be a brief surge, as a cold front approaching from the west will return us to slightly below normal temperatures — 60s highs, 30s/40s lows — for Thursday and Friday. The front will not have much moisture to work with, and it’s possible that showers and storms diminish or die out entirely trying to cross the Appalachians from the west on Wednesday. But anytime a cold front pushes into air this warm, there is some chance of storms, and at least a marginal chance of strong to severe storms with locally gusty winds as the major threat.

A much larger and longer lasting warmth surge is due to effect much of the central U.S. early next week, with summerlike temperatures possibly pushing into some of the same Northern Plains/Upper Midwest areas experiencing record snowpack this week. That will cause flooding problems up there. We probably get in on that warmth surge in time, but its effects may be delayed a few (or several) days by a southern-stream low-pressure system over the weekend and early next week that may cut off and wobble around the Southeast/Mid-Atlantic for a few days. We’ll have a chance of showers for several days, and the combination of the low’s counterclockwise circulation and the position of the high bringing warmth to the central U.S. may circulate east to northeast winds to create a “wedge” effect that holds cooler air in against the Appalachians.

Based on number and distribution of storm reports, Friday actually turned into the worst day of severe weather this week, with numerous reports of wind damage from the Blue Ridge eastward in Virginia and North Carolina, stretching north to New York and south to Georgia. Strong winds were responsible for toppling trees and knocking out power in Henry, Franklin and Bedford counties. There was also an apparent tornado that damaged some structures in Mecklenburg County, Virginia, and another in Georgia. It was was mostly just a quick burst of heavy rain in the Roanoke and New River valleys.

We’ve had winter, summer and spring in the first three weeks of April. Perhaps now we can make some claim to having something of a fall-like weekend, as the cold front will usher in crisp, cool air from Canada for a weekend chock full of largely attended local activities. Highs will struggle to make 60 on Saturday in the Roanoke Valley and eastward, and probably won’t make it at all to the west. Sunday afternoon will be similar, maybe even a bit cooler. By Saturday night and Sunday morning, as high pressure builds in from the west, clear skies and calming winds may allow temperatures to drop to near or below freezing west of Roanoke and some outlying locations and sheltered valleys elsewhere. It may be wise to bring in any tender potted vegetation for Sunday morning, and perhaps Monday morning as well. This general temperature regime of upper 50s to 60s highs/30s-40s lows is likely to continue for several days as occasional frontal passages continue to reinforce the Canadian air mass. Cooler than normal temperatures are expected to dominate the central and eastern U.S. through most or all of the next 10 days.   This kind of weather pattern often shuts off the Gulf of Mexico as a major moisture source for our region, so most of these cold fronts are going to pass through dry or with minimal shower and thunderstorm activity. In that way, it is very much like we often see in October rather than April, which this year, is proving to be a month for all seasons.

UPDATE 5:30 PM, 4/8: A day ahead of schedule, Roanoke hit 80 degrees on Monday, the first 80-degree reading since Oct. 25. Expect highs near or above that level both Tuesday and Wednesday at Roanoke, spreading to even more sites in Southwest Virginia. END UPDATE

The switch has flipped. Just 5 days after Thursday’s fast and furious April snowfall (news NWS-Blacksburg analysis linked), temperatures may well reach 80 degrees at Roanoke for the first time since October. Wednesday is likely to be the warmest day this week, as southwesterly wind flow around a strong central U.S. low-pressure system moving northeastward peaks. Many locations in Southwest Virginia are likely to be near or above 80 on Wednesday (NOAA graphical weather map for Wednesday at left projecting 82 for Roanoke, 79 for Blacksburg) with abundant sunshine. After that, an approaching cold front, pushed by the low moving toward the Ohio Valley and/or Great Lakes, will increase the chance of showers and thunderstorms — some of which may be strong to severe — by late Thursdaya and Friday. Numerous severe storms are likely in the state’s west of us through midweek. This is truly a classic spring pattern, following on the heels of an extended winter pattern. Once the front passes, temperatures will back off a bit for a few days (maybe a day with highs only in the 50s, lows 30s/low 40s), but will recover late in the weekend and early next week back to highs at least in the 70s before another low and cold front approach about the middle of next week. We may be in this repeating pattern of warmups, short cool-downs and chances of showers/thunderstorms in between for the rest of April and much of May too.

Photo by Matt Gentry / The Roanoke Times

Though it never delivered the “big’un,” we will not soon forget the 2012-13 snow season. Thursday added a dramatic final chapter — we presume — to the overtime session of the 2012-13 winter, as Gulf of Mexico moisture caught up to cold, dry air and evaporational cooling allowed snow to reach the surface for several hours Thursday afternoon and evening, providing a rare April snowstorm that was Roanoke’s first measurable April snow in 20 years and the region’s largest in at least 26 years. For the first time since 1971, the Roanoke and New River valleys experienced two significant calendar spring snowfalls, as this one followed on the heels of the March 24-25 “Palm Sunday” snow event. I hope to have some final numbers or maps on Friday regarding Thursday’s snow, but generally, it was a 3 to 7 inch snow across the New River Valley and along the Blue Ridge south of Roanoke, and a 1 to 3 inch snow in the Roanoke Valley and locations just north and east. The amounts did not extend northward as would often be the case, as regions farther north in the state actually warmed more during the day with thinner clouds, and the high pressure system helping to bank the cold, dry air against the mountains began moving eastward out to sea before the bulk of the moisture arrived. So Southwest Virginia was just far enough south to get a solid slug of moisture and just far enough north and west to have that moisture move into very dry air, which allowed evaporational cooling to occur, as heat was removed from the air to turn frozen and liquid precipitation into vapor. Eventually, warmer air aloft did win out, and precipitation turned to rain, but not before several inches of snow had fallen in many locations. I’m glad we were at least talking about the possibility of snow on this blog for a few days before — but the amount of snow that fell in parts of the New River Valley particularly was a surprise for me, just as for many other weather professionals.

One interesting tidbit is that Blacksburg experienced its first measurable snow on Oct. 29, 0.2 inch with Superstorm Sandy, and what we presume to be the last measurable snow on April 4, 157 days later. That breaks the record set in 1970-71, at 156 days, for longest stretch between first and last measurable snowfalls.  Also for Blacksburg, the official 5.4-inch measurement (at the weather service office — more was reported elsewhere nearby) made it the third largest April snowfall on record, trailing only the April 7, 1971, (10 inches) and April 27, 1978 (6 inches) events, just ahead of the 5.3 inches of April 3-4, 1987.  Roanoke’s official 1 inch was the seventh largest April snowfall on a single calendar day — but since the list splits the 1971 and 1987 April snows into two different calendar days,  combining those would pull Thursday’s snow up to the  fifth largest April snowfall.

But, as I noted on here earlier in the week, the pattern is changing dramatically. Thursday evening’s enhanced satellite photo shows the key players. First, near the East Coast, you see the storm system that dumped our snow. Over the western U.S. is a low pressure system that will initiate the southwesterly wind flow that will overtake much of the U.S., leading to much warmer, and more moist, air overspreading the nation. (While the eastern U.S. storm system was snowing on us, the western U.S. storm was spawning a tornado in northern California, of all places). Even farther west is a curve of clouds in the northern Pacific Ocean. This powerful dip in the jet stream will begin to carve out a trough of low pressure over the western U.S. that will lead to several days of southwesterly flow over the central and eastern U.S. This typical spring pattern means quickly rebounding temperatures for us — snow-melting 50s to near 60 Friday, widespread 60s Saturday, many 70s by Sunday and through the early part of next week — plus increased severe weather and rain chances through the central U.S.  So yes, I’m declaring the 2012-13 snow season to be OVER for Southwest Virginia. at least below 4,000 feet in elevation (higher locations sometimes get snow in May.) Spring is on, especially by Sunday. Soon, we’ll be talking severe weather risks, not snow threats.

WINTER STORM WARNING ROANOKE AND NEW RIVER VALLEYS SOUTHWEST FOR 3-7 INCHES SNOW, LOCALLY MORE;  WINTER WEATHER ADVISORIES SURROUNDING AREAS FOR 1-3 INCHES.

UPDATE 7:25 PM: It appears that most locations have switched to primarily rain and sleet mix. Some periods of snow may continue to occur, especially in heavier precipitation bands, but additional amounts are unlikely to surpass an inch in most areas, and what is already on the ground will melt and compact some. END UPDATE

UPDATE 5:45 PM: Winter storm warning now expanded to include most of Roanoke/New River valleys for 3-7 inches of snow, locally more. END UPDATE

UPDATE 5 PM: Winter storm warnings have been issued for Montgomery, Pulaski and Wythe counties, where 4-plus amounts are already occurring in some spots, with a large winter weather advisory covering most of the rest of Southwest and Southside Virginia. It’s obvious now we are getting hammered by what will be a legendary early April snowstorm, likely the biggest since at least 1987. I would not be surprised to see some additional counties added to that warning later, with snow continuing all the way to Interstate 40 in North Carolina. END UPDATE

Photo courtesy of Zach Robinson

UPDATE 4:20 PM: Never mind what I said about not collecting on paved roads — the intensity of the snow has overcome the surface warmth and snow is rapidly collecting on roads in many areas. Snow is falling heavily even in downtown Roanoke now. It looks like the twin-spring-snow of 1971 — one in late March, one in early April –  is repeating 42 years later. END UPDATE

Snow — mixed with sleet and rain in spots — has been rapidly spreading up the Blue Ridge through the New River and Roanoke valleys this afternoon at the leading edge of a large area of precipitation.  Slushy accumulations are already occurring in many spots. Warmer air aloft may eventually change snow over to rain/sleet from the south … we’ll just have to see if, and when, that happens. Paved roads will not be fast to collect snow, but watch out for slushy spots, especially if this continues into evening, or during particularly heavy periods.

I’m still confident this storm system is the caboose on a long-winter train.

UPDATE 3 PM: Snow — mixed with sleet and rain in spots — has been rapidly spreading up the Blue Ridge through the New River and Roanoke valleys this afternoon at the leading edge of a large area of precipitation.  Slushy accumulations are already occurring in many spots. Warmer air aloft may eventually change snow over to rain/sleet from the south … we’ll just have to see if, and when, that happens. Paved roads will not be fast to collect snow, but watch out for slushy spots, especially if this continues into evening. END UPDATE

UPDATE 8 AM: Some overnight forecast models suggest a burst of sleet and/or snow to be a little more likely later today or early tonight, especially along the higher elevations of the Blue Ridge south of Roanoke, and into some of the New River Valley. Do not be surprised anywhere in Southwest Virginia today to see wet snow or sleet for a period of time near or shortly after the onset of precipitation. END UPDATE

Wednesday’s Weather Journal column in The Roanoke Times looked back at the wild contrast in March weather Southwest Virginia had between the sometimes-summerlike warmth of 2012 and the late-hanging-on winter of 2013. Included is a graphic comparing some numbers between the two Marches (a technical problem has blitzed some numbers on the “March Days in Roanoke” part of the graphic — lines should say “at or above” 80, 70 and 60, from top to bottom, then “at or below 32″). It was the widest disparity between March average temperatures in consecutive years — or any month, year to year — since 1960 and 1961, when Blacksburg went from 27.7 degrees to 47.7 degrees and Roanoke went from 34.0 to 51.1. Another unusual feature about March 2013 that I didn’t mention in this column is Roanoke getting nearly 40 percent more measurable snowfall than Blacksburg in a calendar month. Nothing like that has ever happened before in my 14 years here, I am sure.  Spotty snowfall records at both sites would probably preclude tracking how rare that is, historically.

I am declaring Thursday night’s cold rain in Southwest Virginia as winter’s belated last gasp. There may be enough evaporational cooling — precipitation falling into dry air aloft, taking heat from the atmosphere as the moisture evaporates — for there to be some sleet or maybe even wet snowflakes on Thursday afternoon and evening, especially at 3,000-foot-plus elevations. Don’t be stunned, even in the Roanoke Valley, if you see some bouncy icy bits or even an oddly-plopping slushy rain drop. While the storm track along the Gulf and up the East Coast would be almost textbook for a significant to major winter storm in a colder time of year (or even a more strongly blocked pattern like 2 weeks ago), the placement of high pressure (due east, offshore) is not the most conducive for holding cold air in place, even if it were mid-winter. Most likely, all precipitation goes to rain after a couple of hours — but we’ll keep half an eye out for any pockets that can stay frozen longer, especially at higher elevations. Also, later forecast model runs have tended to show the storm a bit weaker and farther east than we were expecting a couple of days ago, so it’s looking like a little bit less of a soaker than we were thinking. Probably more like 1/2 to 3/4 an inch of rain rather than an inch-plus in most places. Temperatures may not make 50 on Thursday and then will likely fall into the mid and upper 30s as it is raining, so this will be every bit a cold rain situation more akin to something that would frustrate snow lovers in January rather than April. Most of the rain should be done by midday Friday, with just a few lingering showers into the afternoon and early evening.

After this storm system goes by, the full-scale pattern change will be under way. We zoom into the 60s/some low 70s areawide by Sunday (weather looks great for racing at Martinsville Speedway), and likely more widespread 70s into early to mid next week. Southwesterly flow will bring occasional bouts of showers in the new pattern — I think a lot of the heavier rain and storms will be focused in the Mississippi and Ohio river valleys to our west, but perhaps some occasional April showers (and a few storms) to bring May flowers the next few weeks. The first shot at this may occur Tuesday and Wednesday.

We had some good discussion on here Monday about the dense fog problems on Interestate 77 in the Fancy Gap area, which resulted in a series of pileups totalling nearly 100 cars and killing three on Sunday. Linked here is an article in today’ s Roanoke Times looking at the long-fought struggle to make that stretch of I-77 safer.

The cold pattern that has dominated the latter half of March is continuing the next few days with below-normal temperatures having been renewed by a cold front, and then, a likely soaking cold rain for Thursday and Friday . But the cold pattern is soon to come to a screeching halt, with spring warmth likely to set for a prolonged period by early next week. Highs will be in the 40s and low 50s through Friday, when normals are in the upper 50s to low 60s. (Lows will be in the 20s to low 30s tonight and Wednesday night, with normals about 10 degrees warmer.) A low-pressure developing along the Gulf and moving up the East Coast (yes, the classic major snowstorm path in winter or a more blocked pattern like a week and a half ago) is expected to bring widespread rainfall approaching or possible topping 1 inch by Thursday and Friday. While it is possible that it will be cold enough on the front end for some spotty sleet or wet snow, especially at higher elevations, any cold air deep enough to support prolonged wintry precipitation will be eroding as the moisture arrives without strong high pressure properly placed to keep colder air in place. Once the late-week rain is past, the subsequent cold front will not be particularly strong, and temperatures will bounce back quickly through the weekend and early next week as a new weather pattern takes hold. We only had one day in the 70s in all of March — next week should easily top that number.