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6/11 Northwest Flow Thunderstorms Discussion/OBS.......


JimCaruso
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Parameters look generally favorable, particularly the NAM but GFS and NAM 4K also indicate TOR. However, I don't see a trigger for initiation in our area. Seems we would have to rely on an MCS approaching within the NW flow. But SPC's overnight outlook seems to have backed off from that idea. The NMM 4K shows a WSW-ENE oriented line to our west at 21z but only one segment seems to survive into southern NJ by 00z. Another line of storms in NYS at 21z appears to hold together pretty well heading into Long Island by 00z. Tried to attach model run images but getting a Tapatalk error :(

Question: all three models cited above show dewpoints in the 70s, but it seems to be an area surrounded by lower dewpoints, as opposed to being part of a contour or moist axis coming up from the west or southwest. At a certain model time the region of higher dewpoints just "appears" as opposed to advecting in. How can this be? Does it represent moisture convergence along the warm front? Or is it model error?

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Parameters look generally favorable, particularly the NAM but GFS and NAM 4K also indicate TOR. However, I don't see a trigger for initiation in our area. Seems we would have to rely on an MCS approaching within the NW flow. But SPC's overnight outlook seems to have backed off from that idea. The NMM 4K shows a WSW-ENE oriented line to our west at 21z but only one segment seems to survive into southern NJ by 00z. Another line of storms in NYS at 21z appears to hold together pretty well heading into Long Island by 00z. Tried to attach model run images but getting a Tapatalk error :(

Question: all three models cited above show dewpoints in the 70s, but it seems to be an area surrounded by lower dewpoints, as opposed to being part of a contour or moist axis coming up from the west or southwest. At a certain model time the region of higher dewpoints just "appears" as opposed to advecting in. How can this be? Does it represent moisture convergence along the warm front? Or is it model error?

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Attempted answer to last question, moisture convergence near a predicted boundary sounds about right. It could also be precip induced.  Either way (looking at ASOS site dew points) last event the models were too moist.  I only looked at a couple of synoptic scale parameters on the nam; looked like the short wave was to our north.  I see the NSSL WRF does bring the line through our area 3-4pm regardless (although its 11z verification is not that great), one of those once it gets started there is no stopping it.  Of the 10 NCAR members, member 7 looks closest. In general they are all too south (some way too much) based on current activity. Member 7 does bring a scattered/widely scattered line of storms to about 40N through 8pm.   The HRRRRRRRRRRX does bring this feature farther south, but its current predicted PA activity is way overdone.  Northwest flow events can be nasty, but I haven't really looked that closer than what I posted here.  If last Sunday I thought most of the activity was going to be from around 40N south, today I would flip that.

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Mesoscale update:

The general expectation continues to indicate the potential for
scattered discrete cells and storm clusters to form this afternoon
to our west and northwest along an outflow boundary from earlier
convection this morning. Higher terrain will likely aid in the
development of scattered thunderstorms to our west-northwest.
These thunderstorms will move east-southeast into Southeastern PA
along with northern and central NJ in the mid to late afternoon
hours. This portion of our region is modeled to become unstable
with CAPE advecting into the region and ranging from 500-1500
J/KG coupled with decent low-level lapse rates. Shear profiles
and helicity continue the theme of potentially sufficient but not
overwhelming with about 30 knots of 0-6 km bulk shear and 100 m/s2
of 0-1 km helicity modeled.

A featured limiting factor will be a CAP which will grow stronger
the further south you get. 700 mb temperatures are modeled to
range from 8-10C across the southern two thirds parts of the
region with increasing CIN as one goes south. This will prevent
any thunderstorm formation across most of the region. The far
northern parts of the region away from the CAP are susceptible to
cloud debris from the ongoing MCS moving across NY this morning.
Thus, it is an all or nothing view waiting on the outflow
boundary to see if it is sufficient lift for scattered
thunderstorm formation this afternoon.

Mesoscale modeling continues to be highly erratic today with
inconsistencies on a run to run basis. However the general theme
follows the thinking above. Storm timing looks to be from mid to
late afternoon across most of the region, lingering into the early
evening hours for portions of NJ. Gusty winds will be the main
severe possibility with these storms. However, a tornado can still
not be ruled out given somewhat looping hodographs and sufficient
shear.

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HRRR looks too hot already, but as Mitch posted, they (all convecting mesoscale models) are all over the place; HRRRX verifying more accurately than the HRRR as of now, but it has developing convection farther sw (what cap?) than any other model I believe later this afternoon. 

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HRRR looks too hot already, but as Mitch posted, they (all convecting mesoscale models) are all over the place; HRRRX verifying more accurately than the HRRR as of now, but it has developing convection farther sw (what cap?) than any other model I believe later this afternoon. 

Dewpoints are looking low as well. BTW won't see me today since I drew the Y shift straw. 

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I'll be out in central Lancaster Co. this weekend. We'll see how it goes.

A glance at the RGEM late this afternoon...

welcome to my neighborhood! Temps climbing pretty fast now and dews up into the lows 60's.
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