I pulled out the weather map at left off the Weather Prediction Center site for Sunday morning, but really, Friday through Tuesday all look very similar, as do the forecast models for the time frame. The blue “H” near us signals the center of high pressure that will be building the next several days. At first, this high will be a “cool” high behind the cold front, keeping temperatures below seasonal norms (70s to low 80s high Wednesday and Thursday, mid 50s to low 60s at night) and bringing dry air in to replace the soppy conditions of the last few days (though there will be a chance of scattered showers and storms on this Wednesday afternoon as the front pushes through). In time, as the high becomes anchored over the Eastern U.S. and offshore, it will help bring a warmup underneath more stable upper air-conditions and slowly increase Gulf of Mexico moisture again with southerly flow on its back side. This will mean warm to hot (lots of 80s, some low-mid 90s possible) and humid weather by next week. While such weather will likely mean chances of daily showers and thunderstorms induced by heat and higher terrain will go up, the high appears likely to deflect large storm systems and cold fronts to our west and north through early to mid next week, at least. The jet stream is finally lifting into southern Canada, with a much more stagnant air flow aloft taking hold over much of the South and East. Bottom line: It’s starting to look more like a summer weather pattern for us rather than a relentlessly wet spring. I’m not saying it won’t be humid with some rounds of showers and storms next week, but it may be a while before the next cold front or well-organized low makes it close to us from the central U.S.

UPDATE 10:50 AM: FLASH FLOOD WATCH FOR ROANOKE VALLEY NORTH AND EAST TODAY. END UPDATE

We’ve gone past “showery” straight into downright “rainy” on this Tuesday as soppy June weather continues. A series of upper-level impulses, plus the presence of two fronts — a stationary front nearly overhead and a cold front sagging in from the northwest — is providing plenty of lift for abundant Gulf of Mexico moisture, producing a widespread area of rain over much of western Virginia into West Virginia, Kentucky, Ohio and Tennessee this morning. Expecting rain much of the day with widespread amounts topping 3/4 of an inch — some of you have already topped that today. Locally heavy rain is possible, and it wouldn’t take much for some flooding concerns. It may become a little more sporadic at some point this afternoon, but there will be a chance of rain off and on into the evening as well. The cloudiness and rain will keep surface temperatures cool and likely eliminate the severe storm risk today — if the sun were to pop out an hour or two this afternoon, it might perk back up.

Other than some morning showers on Wednessday,  both it and Thursday look mostly dry and pleasantly cool for mid June, with highs not getting out of the 70s for many in Southwest Virginia.

It’s raining (and thundering and lightning) again outside my window in southern Roanoke County as I type this, after two nice days this Father Day’s weekend. And there’s more showery weather on the way through at least Tuesday, maybe even leaking a bit into Wednesday. That should come as no surprise in a year running about half a foot or more above normal in rainfall in the Roanoke and New River valleys, and tonight’s showers meaning 12 of the first 16 days of June have had measurable rainfall at the official climate station at Roanoke Regional Airport.  That may well go to 14 of 18 or even 15 of 19 in the next 2-3 days, though this isn’t likely to be a period of solid, continuous rain. A series of upper-air impulses will bring rounds of showers and storms, the first having arrived on this Sunday evening, and more moving through in hard-to-time intervals through Monday and Tuesday. By Tuesday, a cold front approaching from the northwest will enhance the rain chances. The Weather Prediction Center, as of early Sunday evening,  is showing more than 1.25 inches for most of our region through Wednesday evening. It will likely be a little more splotchy and streaky than the map shows, and some indications are that the heaviest rain will be south of our area (the map actually hints at this — the streak of heavier rain right over Roanoke/New River valleys is partly because of the ongoing Sunday night showers as I type this).  Severe storm potential will be dramatically lower than last week’s, with weak wind dynamics aloft, more typical of June, and instability limited by periods of showers and cloud cover. The best chance of getting a few storms with locally damaging winds or some hail would be if one of these impulses moves through after a period of afternoon sunshine.

We’ll dry out by the end of the week, but long term, continue to expect these showery periods every 3 days or so for the next couple of weeks, at least.

An approaching upper-level disturbance, plus heat and increased humidity, will return a chance of thunderstorms to our forecast late this afternoon and into the evening. With temperatures as warm as they will be — 80s to some near-90 readings — the atmosphere will be unstable enough that a few strong to severe storms can’t be ruled out, with damaging wind, once again, as the main threat. Storms will remain a possibility Monday and Tuesday, especially in the afternoons and evenings, until a cold front pushes through to clear the moisture and instability out and leave the latter half of the coming week a bit cooler and mostly dry.

On to the future. We may not be done with squall lines/mesoscale convective systems/derechos with the weather pattern that appears to be setting up. Not this weekend, though. Saturday appears to be totally dry, while Sunday may have a few isolated showers/storms in the afternoon/early evening as moisture begins to build back, cooked by sunshine and lifted by the higher terrain. The next in what could be a long series of cold fronts arrives about Tuesday, with an increased chance of storms accompanying/ahead of it Monday and Tuesday. At this distance, atmospheric shear does not appear to be nearly as strong as with the last couple of severe weather threats, but a cold front pushing into hot, moist June air is often enough for clusters or a line of storms forming that could be locally severe. Something to keep an eye on. Long term, the pattern is looking to take on either a “Ring of Fire” or progressive look, either of which would bring periodic cold fronts and storm systems. The “Ring of Fire” concept involves a central core of hot/dry high pressure — signalled somewhat on the 6-10-day Climate Prediction Center temperature map by the red circle in Texas and Oklahoma — with storm systems circling the perimeter of the high to the north and east, diving toward our region. A progressive pattern would involve less of a stagnant high and more of a west to east movement of storm systems, mostly across the northern half of the U.S., as the jet stream winds continue lifting northward typical of the season. Either way, it will not allow a long-lasting hot/dry air mass to set up anytime soon over us. Continue to expect 2 or 3 days of mostly dry, gradually warming weather, followed by a chance of showers and storms, then somewhat cooler/drier weather.

I’ve been asked a bit about the summer as a whole. We’re already to mid-June and the pattern is unusually active for this time of year. The lean now would be to a near-normal temperature summer — averaging between periods of warmth and cooler weather — and wetter than normal, as storm systems continue to move across the northern U.S. and drag cold fronts through. I think a prolonged heat wave is going to have a hard time getting established in the East, both with the current prevailing pattern and the ample moisture, but wouldn’t rule out a few extremely hot days at some point. Just some thoughts.

Sam Owens / The Roanoke Times

The timing was the worst. The timing of a cold front and associated upper-level “shortwave” was critical for what would come of today’s potential storm threat. Those effects arrived in the middle of a hot, muggy afternoon, instead of the wee hours of a morning. The lack of any morning showers and storms allowed temperatures to rise under bright sunshine and violently destabilize the atmosphere. The front slicing into the hot, muggy air and the shortwave providing additional support aloft was able to easily overcome the effect of downsloping westerly winds blowing over the  mountains. Storms along a squall line erupted, with strong outflow winds that rocked much of Southwest Virginia. It appears one of the worst storms in the line tracked right along the U.S. 460 corridor through the heart of the most populated areas of the Roanoke and New River valleys, where nearly 70,000 are without power. Wind gusts clocked at 68 mph at the Roanoke Regional Airport, combined with wet ground from many weeks of abundant rainfall spelled doom for many fully leafed trees, toppling them into houses and cars.

The squall line is past, the severe thunderstorm watch is cancelled, and three days of warm, dry weather with plenty of sunshine are ahead for Father’s Day weekend.

UPDATE 3:15 PM: Thousands without power after severe storms move through New River/Roanoke valleys END UPDATE

Latest watches/warnings from National Weather Service-Blacksburg

UPDATE 12:30 PM: Severe thunderstorm watch issued until 7 p.m. for the western two-thirds of Virginia. END UPDATE

UPDATE 11:45 PM: The Storm Prediction Center says there is a 95 percent of a severe weather watch being issued today for most of Virginia, with an upgrade to moderate risk extending west to Blue Ridge (Roanoke marks northwest edge of moderate risk zone). Model guidance continues to conflict on whether the line of storms in Kentucky and Ohio cross the mountains intact or diminishes and re-forms to east of the Blue Ridge.  SPC expects instability to be sufficient to maintain the storms crossing the mountains. Arrival sometime between 1 and 4 p.m. likely for Roanoke/New River valleys, if the storms survive the mountain crossing. END UPDATE

This much has become a little clearer: The line to watch for possible severe thunderstorms in Southwest Virginia later today (1 to 4 p.m., or so) is coming out of Indiana and southwestern Ohio, soon to move into central and eastern Kentucky. A new severe thunderstorm watch has been issued for areas downstream in Ohio, West Virginia and Kentucky, edging right to the border of the far southwest corner of Virginia. These storms are associated with a short-wave trough aloft and the advancing cold front. Some storms are also occurring in West Virginia and eastern Kentucky at the end of the larger storm complex to our north, which is generally moving more east and northeast rather than southeast toward us — though some of it may edge into the I-64 corridor in the Alleghany and Rockbridge counties in the next couple of hours. In time, the new line will likely overtake this activity. The big question that remains unresolved is whether or not the new line of storms will move over the Appalachians and eastward this afternoon into Virginia, or whether it will weaken crossing the mountains and a new line will take over east of the Blue Ridge in the Piedmont. Also some questions about how outflow from the ongoing storm complex to our north will affect the new line. Parts of central and eastern Virginia continue under a moderate risk of severe storms today, where instability, deep moisture and wind shear are likely to be maximized for a higher risk of damaging winds and possibly a few tornadoes than we will have in our part of the state.

We’re really down to radar-watching in regard to Thursday’s severe weather threat. The first question is how much of Wednesday evening’s massive storm cluster in the Upper Midwest — finally starting to organize into a “bow echo” at this midnight writing, signalling an increased high wind threat — will be able to make it as far south as part of Southwest Virginia. A lot of forecast models want to keep it north of Interstate 64, and there’s a lot of doubt about how much of it can make it over the mountains anyway. But if part of it does make it over and gets farther south, some morning thunderstorms would be possible.  Coming through at  the coolest part of the day would undoubtedly make the storms weaker than if they had come through in the afternoon or early evening, but some strong gusts may still be possible if they do make it. The second question is how much, and exactly where, and how strong (I guess that’s really three questions) new storms develop as a cold front pushes through during the day Thursday. Some forecast models develop a squall line in West Virginia that pushes through the area during the afternoon. But others are hinting at westerly downslope winds drying out these storms and then re-forming them east of the Blue Ridge as the cold front “jumps” from west of the Alleghany Mountains into the lee trough that often forms east of the mountains in the Piedmont. A morning passage of storms could also cool and stabilize the atmosphere enough that it would be more difficult to re-develop them in the afternoon. So many variables, still. Hopefully this is a little clearer by morning light.

The best course of action on Thursday is to be prepared for severe storms with possible gusty winds throughout the day, though our region is likely to see fewer of them than central and eastern Virginia.

Once we get past this storm situation — Father’s Day weekend looks sunny, warm (80s highs) and dry.

A volatile severe weather situation is setting up for a large area from the Upper Midwest to the Mid-Atlantic and Northeast on Wednesday and Thursday. Southwest Virginia, however, is NOT in the bullseye of this setup. One unusual factor that is changing the playing field from what we would typically expect in early June is an unseasonably strong surface-low-pressure system (depicted at 992 millibars or 29.30 inches of mercury for Thursday evening on 0Z NAM at left) expected to track eastward from Nebraska to near D.C. This low is going to focus a lot of the strong storm dynamics — the best shear, moisture, lift and instability — well to our north. Surface winds backing to the east and southeast just east and north of this low’s track may spur an unusually high tornado risk up that direction — this dangerous possibility may have got lost in some of this week’s derecho hype. A trailing cold front will be cutting into hot (some 90s possible Wednesday in Roanoke and points south and east, maybe even locally mid-upper 90s) and increasingly moist air farther south — its speed and placement will determine a lot about our storm chances.

In short, here are Southwest Virginia’s  three best chances of seeing storms, some of them possibly severe, through Thursday. The picture is a little blurry, but actually leans AGAINST a significant severe weather outbreak in our region.

(1) Scattered storms, Wednesday. This appears more likely to happen to our north near a stationary front. But there may be some chance of storms developing Wednesday afternoon and evening in our region as moisture increases and heating and terrain factors produce instability and lift. Downsloping northwest winds will be an inhibitor for this, at least in the Roanoke and New River valleys and close to the Blue Ridge. Sometimes a “lee trough” develops in this situation that helps fire storms some distance east of the Blue Ridge. UPDATE 8:30 AM: Radar is picking up some storms along I-77  in West Virginia, which may work into some of SW Virginia later today. END UPDATE

(2) Storms moving in from Ohio Valley late Wednesday evening, early Thursday. This is where whatever’s left of one or more MCS’s (mesoscale convective systems — storm clusters or lines) could move in from the northwest, with some locally strong wind gusts possible. The timing of this several hours past peak warming and the tendency for drying out and weakening by downslope flow over the mountains argues strongly against the “d-word” — it’s doubtful high winds could be maintained widely for the length of time necessary to reach us, unlike a certain  100+ degree afternoon/early evening in late June of last year. There is a fair amount of forecast model evidence suggesting this storm cluster will either (a) stay north of Interstate 64, maybe by a wide margin, or (b) fall apart crossing the mountains or (c) both. Sometimes, though, these storm clusters seem to get minds of their own, as they develop their own cold pools from outflow winds behind them that propels them in directions that can defy expectations. A June 29, 2012-like widespread damaging derecho event, however, is HIGHLY UNLIKELY in Southwest Virginia.

(3) New development on Thursday. At this point, this is the most unclear of the three vague possibilities, as it depends on the timing and movement of everything that happens Wednesday. A slower arrival to the cold front could lead to strong storms developing in or even west/northwest of our area and moving through on Thursday; a faster arrival favors development east of the Blue Ridge. Outflow boundaries (mini-fronts produced by winds of old storms), subsidence (sinking air behind the storm that inhibits convection) and other products of Wednesday evening’s storms will also have a big say on what happens Thursday. The strong possibility exists that storms will die or substantially weaken crossing the mountains and then re-fire well east of the Blue Ridge on Thursday, leaving Southwest Virginia totally out of a severe threat.

I put “cold” front in quotes because this is going to be one of those situations where the drier air behind the front will end up helping it get hotter during the afternoons the next couple days — widespread 80s, maybe some low 90s on Wednesday. Tuesday is likely to be mostly dry, though a few scattered afternoon showers and storms are possible on Wednesday with a little more moisture working in. A low and cold front approaching from the Upper Midwest and Ohio Valley on Thursday will carry the potential for a cluster of showers and storms to move southeastward into our region. Exactly what form that will take and how severe it will be will depend on a lot of specific factors not capable of being forecast 3 days ahead of time. Storm clusters moving southeast from the Ohio Valley into our region are a pretty typical summer event. Indeed, we may see more more than one of these situations over the next couple of weeks as a “ring of fire” pattern develops with strong, hot, dry high pressure in the Southern Plains and systems rotating around it on the north and east sides.