Quantitative Precipitation Forecast
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783
FOUS30 KWBC 220830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Tue May 22 2018

Valid 12Z Tue May 22 2018 - 12Z Wed May 23 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNE FVX 25 SE ROA 10 SW MWK 10 S MRN 20 ENE GSP 15 ENE AGS
20 E EZM 15 NW ABY 30 E GZH 20 WSW MGM 15 NE BHM 15 W DNN
35 WSW TYS 45 ENE TYS 10 S VJI 10 ENE VJI 15 SSW I16 25 WNW I16
50 SSE ILN OSU BKL 15 ENE CGF 20 ESE ERI 25 SSW BFD 10 NNE AOO
15 ESE AOO 20 WSW THV MTN CGE 25 SW SBY 20 ESE FYJ 10 SSW OFP
10 NNE FVX.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SW K2D5 25 SSE K2D5 VVV 15 NNE 1G2 KACQ 15 ENE CCY 10 W ALO
15 ENE SLB 25 NW LRJ 35 SE MHE 20 ESE 9V9 15 WNW ICR 40 SE IEN
15 N LAR 15 ENE RWL 15 ENE BIT 10 W BIT 40 ESE BPI 40 NE JAC P60
10 SSE LVM 40 E HLN 35 SSE GFA LWT 50 ESE LWT 55 ENE BIL
50 SSW MLS 25 SW BHK 20 ENE BHK 25 E DIK 20 NW KY19 45 E BIS
40 WNW K2D5 15 SW K2D5.


...East...

The wet pattern will continue today across much of the east. A
moderately unstable airmass, moderately anomalous PWATs, and broad
troughing will support scattered to widespread convective
activity. A few forcing mechanisms within the broad troughing will
help focus convection. Will have stronger synoptic forcing and
wind shear across the central and northern Mid Atlantic, being
closer to the mid level shortwave and upper jet to the north. Thus
would anticipate convection will be a bit more organized here,
probably generally in small linear segments. Will also have whats
left of the backdoor front that is lifting north as a warm front,
which should locally enhance convergence. Instability will drop
off north of the front, which should cap rates there, however
along and south of the front will have ample instability to
produce locally heavy rates across portions of northern VA into
MD. Storms should form near the terrain of WV/VA/MD this afternoon
and progress eastward. Generally expect storms to be progressive
in nature, limiting the overall flash flood threat. Although a
localized threat probably still exists given the saturated
conditions over portions of the central Mid Atlantic. Also likely
to have storms form along the actual cold front further west over
the OH Valley, and move eastward into portions of western PA.
These should weaken after sunset as they head into the terrain.
Will cover both of these areas with a broad Marginal risk. Gave
some thought about going with a Slight Risk across portions of
northern VA into the DC area, as note some higher HREF
probabilities of localized 3" amounts here, and recent HRRR and
Experimental HRRR runs are wetter as well. In the end stuck with
the Marginal given some uncertainty on the exact evolution and the
expected progressive linear nature of storms. However will need to
monitor through the day, as it would not take much cell merging to
cause issues over this saturated area, and based on trends through
the day, an upgrade to a Slight Risk may be needed around the DMV.

Across the southeast a lingering mid level shortwave will once
again act as a focusing mechanism for storms this afternoon. This
feature is currently over southern AL, and is responsible for the
small scale training shower activity over southeast AL. With
diurnal heating, anticipate a broader area of scattered convection
to form around this wave across portions of AL and GA. As with
past days, not expecting this activity to be organized, and cells
should generally be pulse in nature and thus short lived. However
briefly intense rates are likely, and where cells are able to
merge extending rainfall duration, a localized flash flood risk
exists. Activity should diminish after sunset, with a more
localized band of showers likely near the center of the wave
(similar to this morning).

In between these two areas, the mid/upper level focus for
convection is less evident. However seems likely that we will
again see cells form over the terrain of the western Carolinas,
eastern TN and southwest VA with diurnal differential heating.
These cells will tend to drift eastward off the terrain with time.
Normally this activity would not really pose a flash flood risk,
but given the saturated conditions over portions of this area, a
localized threat likely exists once again. May need to watch
northeast GA into western SC and southwest NC a bit closer this
afternoon. Some chance that we see a bit more enhanced cell merger
threat here, with cells moving northward on the periphery of the
wave over AL/GA possibly merging with cells moving east off the
higher terrain. If this were to occur then may see a bit more
focused flash flood risk develop. Confidence is not high on this
though, and will just need to monitor through the day.


...Central and Northern Rockies into the Plains...

The approach of the mid/upper level trough will increase
convective chances across the Rockies into the Plains. Upper level
flow will become very divergent over WY by this afternoon, which
combined with increasing instability,and the presence of a low
level boundary, will result in scattered convective development.
Instability should be high enough to fully take advantage of the
anomalous PWATs and produce some locally intense rates. Storms
will generally be progressive in nature off to the north, however
could see multiple rounds of storms given the favorable synoptic
environment. Thus at least a localized flash flood risk probably
exists across WY into southern MT. Eventually these cells should
take a turn to the right and move into the northern plains, with
one or more convective clusters likely. Low confidence on how this
evolves, but locally heavy rains are possible into SD/ND as
convection moves across. Storms should generally be progressive,
although one area to watch may be southwest SD. Some signs that
storms that initiate off the Black Hills could merge with
convection moving northeast out of WY/NE. Should this happen a
more focused flash flood risk could develop.

As we head into the overnight hours appears to be a pretty good
setup for an axis of elevated convection across portions eastern
SD into southwest MN and northwest IA. 850 mb moisture transport
increases and persists through the night, with this elevated
activity likely focusing on the nose of this feature. Storm motion
vectors appear to have a large component parallel to this moisture
convergence axis...suggesting that storms may tend to regenerate
and train for a period from northwest to southeast. Would appear
that both instability and PWATs would be high enough to produce
some pretty heavy amounts should this materialize. Still a good
amount of uncertainty though with both the organization of this
activity and the latitude it sets up at. Generally favored an axis
close to the 0z nam nest and ARW2 for the WPC QPF, although likely
will need to make some adjustments to this as things become
clearer by later today. Given the uncertainty and rather dry
antecedent conditions will carry a Marginal Risk for now, although
some chance a Slight Risk could be required on future updates
depending on trends. Either way, something to watch, as certainly
could see some flash flood potential with this setup.

Chenard



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