high risk

Thanks for digging that up, Yoda! I was surprised to see that severe gusts got closer to DC than I remembered, although the death of the system as it moved across northern VA is pretty clear in the reports.

friend in Crofton reported 5 min of marble hail storms now firing again to the west and northwest

August 9 2000. A derecho was blasting through the Ohio Valley, and it was heading our way - on target to arrive during rush hour and potentially cause a disaster. SPC went with the rare PDS severe box, calling for wind gusts exceeding 80 kt. We didn't have CAMs back then, so it was all about environment, and it seemed like we had decent cape and shear in place. But we've learned over the years that these wind-producing MCS events only survive crossing the mountains when the lapse rates are huge, and they weren't that day, and the thing got totally ripped up coming over the Appalachians. It arrived as a line of showers with winds gusting to 25 kt or so. Granted, had it arrived at full strength during rush hour, you would have had fatalities and chaos, but it was a massive letdown. URGENT - IMMEDIATE BROADCAST REQUESTEDSEVERE THUNDERSTORM WATCH NUMBER 666STORM PREDICTION CENTER NORMAN OK1219 PM EDT WED AUG 9 2000THE STORM PREDICTION CENTER HAS ISSUED ASEVERE THUNDERSTORM WATCH FOR PORTIONS OF DISTRICT OF COLUMBIA MARYLAND SOUTHERN PENNSYLVANIA NORTHERN VIRGINIA CENTRASL AND EASTERN WEST VIRGINIAEFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 100 PM UNTIL700 PM EDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION...DESTRUCTIVE WINDS. ALSO...HAIL TO 1 INCH IN DIAMETER...THUNDERSTORM WIND GUSTS TO 95 MPH...AND DANGEROUS LIGHTNING AREPOSSIBLE IN THESE AREAS.THE SEVERE THUNDERSTORM WATCH AREA IS ALONG AND 70 STATUTE MILESNORTH AND SOUTH OF A LINE FROM 60 MILES WEST SOUTHWEST OFMORGANTOWN WEST VIRGINIA TO 10 MILES EAST SOUTHEAST OF BALTIMOREMARYLAND.REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS AREFAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA.PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENINGWEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLEWARNINGS.OTHER WATCH INFORMATION... CONTINUE...WW 665...DISCUSSION...INTENSE BOW ECHO OVER SERN OHIO IS EXPECTED TOCONTINUE EWD AT 55 KT ACROSS THE APPALACHIANS WITH WIDESPREADDAMAGING WINDS LIKELY. ACTIIVTY WILL REACH THE BALTIMORE/WASHINGTONDC AREA AROUND 5 PM.

I'm in for a couple of severe reports somewhere in northern VA later today, but it will be very isolated.

Agree that simulated radars don't look great, although I would expect the reflectivity signatures to look modest, given the likely mode of low-topped cells. The soundings actually look unstable enough, but it may be too much dry air in the mid levels that suppresses widespread activity. Agree with SPC that areas northwest of DC probably have the best shot.

6z NAM nest simulated radar looks interesting for Friday late afternoon in northern VA, but that model tends to destabilize too quickly behind morning convective systems. Instability will be the key tomorrow. We usually fail at getting clearing behind morning systems, so the pessimism is justified, but it does occasionally work out here.

This intrigues me. Usually when we have the strong shear, the system is so strong that we get widespread rain/convection which destroys any chances for discrete cells. But all of the widespread rain will be moving out early, and it's pretty clear that we'll dry slot behind it. If we can get any heating Friday afternoon, there should be just enough lift on the front and perhaps with an embedded short wave to generate some discrete cells which would be in an environment with favorable shear.

well, the storm reports map is a kick to the groin. Huge totals of wind damage north and another to our south. SPC tried to play "connect the two threat areas" with their watch box, and it didn't work out. again, hi-res modeling continues to make huge strides in assessing threats. The parameters showed a "maybe" threat for our area (strong shear but instability being eliminated by the hour during the evening), but the hi-res guidance was emphatic that we would not see severe. I won't sit here and say that hi-res guidance is always going to nail it, but when multiple models show a consistent signal, it should be given a fair amount of weight.

I would say that they emphasized the Blue Ridge and Catoctin Mtns and points west, and I fully agree with that. They also commented that the setup further east had more uncertainty. That's pretty fair, as it's unwise for them to write off the event with us in slight risk.

we're like brothers in arms on this event. I like the chances well to our west and northwest, but I don't see much hope at all for svr in the DC/Baltimore area. Instability is modest to begin with, and it will drop off as we go into evening. The hi-res guidance looks consistently unexcited for us, and I can't ignore that.

To see why the HRRR (and other hi-res guidance) hates our area later today, here is a forecast sounding for 6PM in south-central Montgomery Co. The dew point was in the upper 60s earlier in the day (with cape in the 1250 range), but it mixes out the low-level moisture later in the day, dropping the sfc dew point and killing instability. If it's wrong about this mixing, then we'll be in business. Otherwise, meh.

Cheers to WW007! This case is a great example of the evolution of severe weather forecasting from basing it off of forecasted parameters to forecasting it off of details in hi-res guidance. It has previously been the latter, and we're moving towards the former - kind of in the dreaded "in between" zone now, figuring out how much to trust when the models show "meh" when the parameters scream "woohoo!" (or vice versa). We saw this on the FL/GA high risk bust earlier this year: the parameters were insane, yet the models showed weak updraft helicity signatures. The models were correct. But it would be unwise to say that we can always trust the hi-res forecasts. So today, the parameters here look good, but the reflectivity forecasts are uninspiring until you go further north and northwest. It *may* be related to the models mixing out some of the low-level moisture. If you look at NAM nest or HRRR forecast soundings, the dew points drop from the upper 60s this morning to the lower 60s by the end of the day. If that's right, a big event here won't happen. If the mixing is overdone, then it could be game on.

That's just any hi-res model: NAM nest (3 km), HRRR, NCAR ensemble, Hi-Res Windows....

I'm on board with KMLWX (which is appropriate, given the regional severe scale). Parameters are good, but the CAMs seem to hold our part of the line back to the west, perhaps due a veering LLJ. The experimental HRRR extension, though, does show an axis further east along which the storm could fire. it's hard to ignore that unfavorable signal in the CAM. Otherwise, I'd be in for sure.

Latest NAM nest run and last few HRRR runs still like some initiation around 22z mainly south of the DC Beltway. Hard to ignore that signal, although it wouldn't surprise me if nothing goes up - the NAM nest develops storms nicely, but it looks like it's running high with the dew points in the lower levels. Still a highly conditional threat.