purduewx80

flash flood watch includes the city now. a sort of triple point low will slowly move across this evening, helping to focus showers and storms. some of the hi-res absolutely nails the area w/ some 4-6", locally higher, amounts. thinking more along the lines of 1-2" w/ local amounts to 4" (favored in NJ).

3.3" DCA past hour. KDCA 081352Z 11008KT 1 1/2SM R01/3000VP6000FT +TSRA BR OVC010CB 23/22 A2989 RMK AO2 RAB1255 SLP122 OCNL LTGICCG NE AND SE-SW TS NE AND SE-SW MOV S P0330 T02330222

yep, few lightning strikes out that way. that area had some of the best heating early on. HRRR has been awful w/ this pattern past few days, looks to be a repeat today.

the 12z OKX sounding shows a convective temp of only 83F. Won't take much more heating to get showers/storms to pop today. seabreeze fronts will probably bring the first cells of the day to the area early this afternoon.

PWATs will be over 2" tomorrow and we have this weak upper feature currently over WV moving overhead - the past 2 days this has produced widespread PM storms over the Mid-Atlantic and interior NE. Development has been occurring by late morning, so I think we could see storms off/on throughout the afternoon tomorrow. The frontal feature to our N doesn't move through until late evening.

highest chances look to be 4-7PM for the city. maybe something isolated as early as ~2. still some variability in timing, so we could see that move up or back by 1-2h.

It's clearing out pretty quickly behind this first batch. it'll be important to watch trends over the next hour or two to see if that line approaching northwest jersey makes it here. vis sat does show towering cu building ahead of it, so areas that have been untouched may still get in.

no. there is better instability down there because there has been heating most of the day.

Keep in mind it was the warm front that produced the flooding in NJ/PHL last evening. The small circulation currently in southern PA will help produce fairly numerous storms along the warm front this afternoon, which will likely be right over the city. The setup is different from yesterday and the day before, but I think we should see fairly numerous thunderstorms ~2-7PM. Already getting development in NJ/PA.

There's decent synoptic support for convection now in the DC area to ride the northward-retreating warm front into the metro later this evening. The 500 vort and associated jet max are impressive for this time of year: These features have strong lift associated with them, so they'll take advantage of seemingly minor instability. We still have several hours of sun and a developing seabreeze behind this morning's low to help bring the effective warm sector back to most of the Jersey Shore and perhaps the south shore of LI. It's probably too little too late to get the best activity into the city, but I think a good chunk of central/southern NJ is in for a good thunderstorm this evening. Can't rule out some good cloud-to-cloud lightning south of here around 9 or 10 this evening.

Near-shore water temps in the low to mid-60s and dry air just above the surface are a negating factor, but there is a narrow corridor across the Delmarva and NJ where better moisture transport and higher instability will occur. With the good heating, a seabreeze will shift inland, bringing higher dew points (50s possible much of the day where temps warm well into the 80s) but milder temperatures. 15Z HRRR sfc-based instability valid 22Z (max just inland from the coast where upper 70s T and upper 60s Td align): 15Z RAP (KEWR at 23Z) and HRRR (south Manhattan at 23Z) both indicate a potential for isolated severe cells (most likely near sea-breeze fronts):

Models aren't handling this complex in western PA well at all. KCLE radar shows a rear inflow jet and loops indicate a mesoscale vort that should continue eastwards across PA towards us. It's interesting the SPC meso-analysis indicates almost no instability, yet new cells keep firing. I think w/ good heating ahead of it, and an influx of 65+ Tds up the NJ shore this afternoon, we should see a flare up over and especially on the east side of the mountains. Timing out its current motion would put scattered cells just west-southwest of the city by 5, reaching NYC 6-7PM. We'll have to see whether the hourly updating models pick up on this feature. So far almost no 12Z guidance is doing well.

Depends which model you look at. GFS and 18Z RGEM are weaker than NAM and a number of the other hi-res models. Really comes down to moisture return tomorrow since it looks like there will be plenty of heating. If we get dew points surging into the mid-60s or higher by late afternoon/early evening, we should be good for at least isolated cells after ~5PM. Otherwise, the low-level jet will do it later in the evening.

yeah, hopefully the midwest will get in on some drier conditions. looks especially rough across TX/LA the next few days. pretty high likelihood for a swath of 6-12" inland from the coast. the blocking and tendency for a beefy southeast ridge of late is worrisome, but i'm hopeful they'll get in on a wetter pattern across the southeast by mid-month.

Yeah, don't see why marginal won't cover NYC and southern New England on subsequent updates. Flow is fairly weak in mid-levels and lapse rates aren't like they were w/ the SI hail event last week, but there is some impressive moisture/theta-e return and focus in the region. Could maybe see some brief spin-ups along with isolated wet microbursts, in addition to the tropical downpours.

Reference: tomorrow night's heavy rain threat - with the lapse rates pointed out by @forkyfork and 1.75 - 2" precipitable water available, this low-level jet will focus multiple rounds of convection in the region.

Not as confident on where the best precip chances set up after tomorrow night's deluge, but it looks increasingly likely we'll be backdoored from late Thursday through the weekend. Most guidance suggests we'll be drier and probably sunnier this round. My guess is that if wet weather doesn't return Sunday, it will Monday...lingering into much of next week. That trough @bluewave posted above will produce stormy, southwest flow here as long as it's in the Great Lakes. Once the trough axis is as far east as the ECMWF above, wet weather would probably settle into the Southeast and southern Mid-Atlantic.

low level lapse rates enable strong winds to reach the surface. mid-level lapse rates are what help w/ hail production.

tomorrow looks gorgeous....most of us should get into the lower 80s, even w/ a PM seabreeze on LI.

couple areas of rotation in that warning....could see some brief spinups.

absolutely, 1-1.5" should be common.

NJ counties adjacent to the Delaware River should have the highest severe wx threat today, including tornadoes/hail. HRRR boundary location looks pretty good later, continued heating under partial sun will help slow the backdoor's progression this afternoon. Winds on the cool side of it will trend to the SE, helping enhance low-level helicity given strong and deep westerly flow aloft. Curious to see how SPC/OKX handle watches later.

backdoor

Day 1 outlook from the SPC now has NYC within the enhanced outlook. Today we have a backdoor cold front that will stall just S-W of the city during the afternoon, likely enhancing some hail/tornado threat in NJ as scattered supercells develop ahead of a bow echo. Forecast models are in surprisingly good agreement on this scenario, and it makes sense seeing the approaching shortwaves and clearing ahead of them. I think wind will be the primary threat in the city since the inversion will be shallow and the line will be moving through during the convective max. That boundary may also focus cells nearby and lead to flash flooding in areas that saw the heaviest rain yesterday evening.

agreed, possibly tomorrow's too. @NJwx85