batmanbrad

not only severe, but WPC says flooding could be a concern too: Mesoscale Precipitation Discussion: #0227 (Issued at 635 PM EDT Mon May 05 2025 ) MPD Selection Mesoscale Precipitation Discussion 0227 NWS Weather Prediction Center College Park MD 635 PM EDT Mon May 05 2025 Areas affected...portions of eastern VA into the DMV and far eastern WV Concerning...Heavy rainfall...Flash flooding possible Valid 052235Z - 060300Z Summary...Training convection will be capable of hourly totals of 1-3" with short-term (3-hr) totals as high as 2-4". Isolated to scattered instances of flash flooding are possible (and may be locally significant in sensitive urban/mountainous terrain). Discussion...Radar and GOES-East satellite imagery indicate the proliferation of convection across portions of eastern VA/NC over the past several hours, moving fairly rapidly (~25 kts) towards the N-NE within nearly unidirectional flow on the eastern periphery of a large, deep layer (850-200 mb) closed low centered over IL/IN/KY. The mesoscale environment in the vicinity and downstream of the aforementioned convection is characterized by SBCAPE of 1000-2000 J/kg, PWATs of 0.8-1.3" (between the 75th and 90th percentile, per IAD sounding climatology), and deep layer (0-6km shear) of 20-40 kts (per 22z SPC SFCOA analysis). The strongest cells have been capable of impressive instantaneous rainfall rates of 3-5"/hr (per MRMS estimates), which has resulted in estimated hourly rainfall totals of up to 2.5" (where deeper convective cells have been able to occasionally train, mainly to the east of I-95 in eastern VA). Recent hi-res CAMs have not handled the evolution of convection particularly well, and recent observational trends (including continued overshooting/cooling cloud tops via GOES-East imagery) suggest that localized hourly totals of 1-3" will continue to manifest farther north (into the more sensitive DMV region) with storm propagation (as a pool of 1500-2000 J/kg of SBCAPE remains untapped). While both of the hourly updating CAMs (HRRR/RRFS) are handling the convection poorly, the 18z HREF suite does still give a good idea of the potential for excessive rainfall through 03z (with 40-km neighborhood probabilities for 2" and 3" exceedance of 20-30% and ~10%, respectively). Given locally sensitive terrain (per FFGs as low as 0.75-1.50" and 1.50-2.50" for 1-hr and 3-hr periods, respectively) over urbanized terrain along and near I-95 and over portions of the Appalachians in the vicinity of northern VA, western MD, and far eastern WV, isolated to scattered instances of flash flooding are possible (and may be locally significant, should 2-4" totals occur over the most sensitive localities). Churchill ATTN...WFO...AKQ...CTP...LWX...RAH... ATTN...RFC...MARFC...SERFC...NWC... LAT...LON 39827743 39597667 39037647 37767669 36447699 36217739 36777738 37437742 38277778 39617833

might have been a "false alarm", but I was getting a TVS on the cell NW of Leesburg on RadarScope for a bit, but it seems to have disappeared.

I'm sure DT loves this run, both for it being a jack in his area and because he worships the Euro.

looks like a modest band about to cross I-270 in Moco... and perhaps one more moving east of Winchester, that's about it.

don't forget... Georgetown 0" snow, all rain

now ya got me humming "Kind Of A Drag" by the Buckinghams.... LOL. I'm originally from NJ (not as far northwest as Walt is right now) but I skiied a lot in Vermont and New Hampshire and would always read Walt's discussions with mucho interest and enthusiasm especially during winters.

I bet nobody would have thought the (healthy) storm we are getting tomorrow would potentially serve as an "appetizer" for what the GFS is advertising for next weekend!

and for all the optimists, here's the corresponding 10% chance map, also has been increased!

along with countless references to the infamous DC Snowhole

nice fatties near the end of this band coming through Gaithersburg (near Laytonsville)

some may have thought this was an underperformer but at the MoCo Airpark they've measured about an inch as of the 2PM obs, and certainly we could squeeze out another 1/4 to 1/2 inch here... since they'd basically been predicting 1-2" areawide today, that seems pretty much on target.

radar seems to be showing another line about to (slowly) enter western MoCo soon - maybe not (yet?) quite as potent as the last line that went through recently, but we'll see. Could even have a bit of a training effect as the heavier precip is sliding SW to NE as the line itself gradually pushes east.

over 14 inches today so far as of the 9PM observation - likely has or will be setting a daily rainfall record since likely to have heavy rainfall right through midnight, wonder what the record was going into this storm. edit: 17 1/2 inches today as of 10PM

ugh the HAFS-B model has this just west of the Tampa area with SLP in the mid-920s on its 12Z run as of early morning Wed. 10/9

18z GFS so far coming in weaker and further south, typical model back-and-forth continues

small consolation to all the affected areas/population, but this one's a no-brainer - "Helene" will surely be retired so no more systems with that name will happen.

any chance that heavy stuff near and to the west of Richmond might make it up into the DC metro area if it continues to move N or NNE this afternoon?

I'd say even more than that; previous NHC advisory had LF around Apalachee Bay, now the cone center looks to be around Port St. Joe/Apalachicola area, I'd say 40-50 mile west shift?

Dr. Cowan just posted one of his excellent video discussions on the tropical tidbits site, highly recommended. Said he would have another one tomorrow.

HWRF (also now using Invest 97) bottoms out around 926mb at hour 78 in the eastern Gulf, heading generally NNE towards Apalachee Bay area. edit: comes ashore near the Apalachee Bay region midday 9/26 in the low/mid 930's

TVS signature showing up on Radarscope for that cell a bit SE of Purcelleville

I'm in Gaithersburg (but very close to Laytonsville), have had some good solid downpours for a while though RadarScope seems to tell me the heaviest pixels went to the west/north of me. Stuff seems to be trying to backbuild somewhat down towards the Rockville area so if it keeps training SW to NE we might get more here. Nothing really severe (even though we were warned), some moderate wind but not a whole lot of T&L so far. Edit - actually most of the heavier activity isn't in Rockville, it's east of Poolesville, just north of Darnestown and in parts of Germantown/Gaithersburg, all moving NE. Then it's a break until the next batch out near Front Royal which could get here if it holds together.

usually CWG and/or NWS Sterling produces one after the event is over; for now best would be to check the reported totals in the PNS from LWX (last one was updated around 10:30AM this morning so its totals are not yet final, especially since some of the reports are from early in the morning).

looks like a couplet on the velocity scan, now east of Damascus, heading for Mount Airy area.

my phone just beeped, TOR warning for central Moco (I'm in NE part of Gaithersburg, near Laytonsville). Heading towards Damascus, says the warning.