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Spring severe weather chatter


Ian

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   yes, so the big early day MCS may now go well to our north, but we'll have a better shot at svr later in the day with the second short wave.    12z NAM suggests that a secondary sfc low may form over northern VA Thursday afternoon which would add to the potential.

 

 

For thursday, itseems like that low keeps trending a bit further N across PA lately, no?

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   yes, so the big early day MCS may now go well to our north, but we'll have a better shot at svr later in the day with the second short wave.    12z NAM suggests that a secondary sfc low may form over northern VA Thursday afternoon which would add to the potential.

 

hmm, i like the sound of that

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12z GFS SBCAPE at 18z tomorrow is the lulz at KEZF... > 6000 SBCAPE

 

Also, are we advecting in the remnants of an EML tomorrow afternoon?  Reason being is that I see a lot of mid-lapse rates from 7.0 C/KM to 7.5 C/KM at 18z and then around 7.0 C/KM at 21z

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     The problem with this sounding is that the software is taking pressure level data and then sticking in the 2-m temperature and dew point and doing its own computation.    The GFS has had a widespread problem this spring of pumping a lot of moisture back right into the lowest level during and just after peak heating, and it's really, really inflating 2-m dew point values.    (The NAM has been doing it to a lesser extent;  the soil got pretty wet in May.)   

 

       The actual GFS forecast data for IAD at 18z Thursday has a dew point of 73 at the lowest model level.   And that computation yields barely half of the amount of sfc-based cape that this plot shows.  You can clearly get a sense looking at the plot that if you reduce the dew point at the ground by 2C, it's going to bring the sfc-based cape down quickly.    It's not your fault for posting this, since the product is out there;   I just want to caution people to beware of the artificially large values caused by the computation using 2-m values to represent the surface.    If the forecast sounding has that kink in dew point right at the surface, big values of cape are likely not properly representing things.

 

 EDIT:  to be clear, even without 6000+ cape, it's still a very nice severe wx sounding with a respectable wind profile

Impressive (IMO) sounding for KIAD at 18z THURS from the 12z GFS:

 

 

 

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LWX afternoon disco for Tuesday:

 

Tuesday remains a bit of a question. Guidance has generally slowed
the progress of the front down and it appears to wait until late
in the day or the early evening before clearing the area. Indices
are fairly robust but westerly flow would suggest that the models
are overdoing convective coverage. Any storms that do manage to
fire could go severe...but think coverage will be below 50
percent.

Front clears the area Tuesday evening and a drier and slightly
cooler air mass arrives behind it.

 

For Thursday:

 

Wednesday will be a day between systems with high pressure nudging
south from Canada and another dry air mass. Temperatures won`t
drop that much however with readings well into the 80s. Warm front
pushes into the area from the southwest on Wednesday night with
showers and perhaps some embedded thunder. Some steady rain is
possible Wednesday morning as the front pushes through. Most
guidance now has the front lifting north of us and bringing a very
moist...warm and unstable atmosphere across our area for Thursday
afternoon. With good shear and instability and the approaching
cold front to our northwest...would think we will have a severe
weather episode during the afternoon and evening.
With PW`s so
high also would expect some heavy rain and if any training or
repeat storms occur...flash flooding might be a concern as well.
Lows Wednesday night will be high 60s to low 70s with highs
Thursday in the high 80s to low 90s.
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so I guess this doesn't go into the June 21/23 thread, but SPC has some of the area in marginal threat for today.    HRRR does show around 1500 src-based cape for a nice chunk of the area along with moderate deep layer shear, and it has a few modest cells around this afternoon.    Tend to think that most activity will stay sub-severe, but the 5% probabilities are probably not a bad idea.

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We might see that line extend to the south a bit (hopefully). The SPC meso maps on the frontogenesis maps have some purple back near the I-81 corridor. Shear is probably passable as well - seems to have increased a touch in the last few hours. CAPE  jackpot remains centered right around and just to the SW of DCA. SBCAPE running 2500-3000 it seems. DCAPE also still decent at 1100. 

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New cell near Baker, WV. Question now becomes how strong this stuff gets as it comes out of the mountains. Looks pretty mundane for now unless you're up in PA. But it's entering better instability now. Shear is passable as mentioned above. Mid level lapse rates suck as usual ;)

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