Quantitative Precipitation Forecast
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FOUS30 KWBC 220034
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
833 PM EDT Mon May 21 2018

Valid 01Z Tue May 22 2018 - 12Z Tue May 22 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ESE MMCS 20 E TCS 15 S 4SL 15 WNW ALS 20 NNW VTP 20 NNE RTN
20 ENE CAO 25 WSW DUX 20 SE PVW 10 WSW SNK 65 SSE MAF 35 SW 6R6
80 S E38 60 SW MRF 70 SSE MMCS 15 ESE MMCS.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 W KMIU NQI 30 NNW ALI 35 WNW VCT 35 NW VCT 20 WSW CLL
30 S PSN 10 SSW OCH 15 WNW JAS 30 SSE JAS 15 NW KVBS 30 SE KXIH.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 SW APF 45 WNW TMB 20 SSE HST 40 E MTH 20 ESE NQX.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 W LBB 45 S LBB 25 SE MDD 30 N FST 20 SE GDP 35 ESE ALM
25 SW SRR 40 W 4CR 25 NNW CQC 30 SSW TCC CVS 35 W LBB.


...01 UTC Update...

Once again, made no significant changes to the on-going forecast,
although a few nudges were made to the boundary of the marginal
risk over parts of the Appalachians to northern North Carolina and
to the boundary of the slight risk over New Mexico.  Some of those
adjustments were made to better fit late day convection as shown
by recent satellite and radar imagery as well as most recent runs
of the HRRR.  The HRRR Time Lag Ensemble agreed with the
downward-trend between the 00Z and 03Z time-frame...with
probabilities of 1 inch- and 2 inch- per hour rates dropping to
less than 30 pct.  The New Mexico area still has time for up-scale
growth well into the early morning hours as moisture from the Gulf
of Mexico continues to be drawn into the area.

Bann


...15 UTC Update...

Overall, no large-scale changes to the previous outlook.  Made
some minor adjustments based on recent satellite and radar trends,
as well as guidance from the recent runs of the HRRR.  These
included a slight westward expansion of the Marginal risk over the
Arkansas basin, as well as over the southern Great Lakes region.
Also expanded the Marginal risk to cover a great portion of the
Florida peninsula.  Guidance shows locally heavy amounts across
these areas which may pose short-term runoff concerns today.

Pereira


...New Mexico into Texas...

Convection will develop this afternoon across western NM along a
tightening low level convergence axis and broadly divergent upper
level flow. Activity should be rather widespread in nature, and
will probably form into one or more linear segments as it
propagates eastward with time. Storms are expected to generally be
progressive in nature, although two periods of slower
movement/training are possible. First near initiation time over
central NM, where storms may linger near the terrain features that
help initiate them. Then later at night an increase in
southeasterly 850 mb moisture transport is expected. As this feeds
into the eastward moving convection, storms may try build
southeast into the low level flow and better instability. This
motion may allow for some brief training to occur. PWATs increase
into the very above normal range across this area, so any repeat
cell or training could result in some flash flood concerns. Will
maintain a Slight risk across portions of southeast NM into far
west TX. Some lingering uncertainty with the magnitude of the
southeasterly 850 mb moisture transport into this area, which will
likely impact how organized and persistent convection is as it
moves eastward...so will need to monitor trends through the day.

Will also carry a Marginal risk across portions of southeast TX. A
good model signal that convection will develop with diurnal
heating along a weak boundary forecast just inland from the coast.
Not expecting much organization to this convection given weak
flow. However PWATs will remain high, and the weak flow will allow
storms to be slow moving, with the boundary possibly focusing
cells long enough for some higher totals. Not expecting widespread
flash flood issues, but a localized threat is possible, especially
across areas that saw some heavy rains yesterday locally lowering
FFG.


...Lower MS Valley into the TN/OH Valley and Great Lakes region...

Another day of active convective weather stretching from the Lower
MS Valley northeast into the Great Lakes region. This activity
will develop in a broadly convergent low level pattern, with ample
moisture and instability for locally heavy rains. Across portions
of MS, eastern AR, and western TN/KY flow will generally be weak,
and thus convection should generally remain unorganized and more
pulse in nature. With that said, this will be the axis of greatest
instability (> 2000 j/kg in spots), and thus anticipate some
strong convective cores to develop. As we have seen the past few
days, while most of these storms will be short lived and brief,
some cell mergers along outflows will likely occur...leading to
localized areas of 3"+ rainfall and isolated flash flood concerns.
Will carry a Marginal risk here.

Further north across portions of IN/OH, flow and forcing will be
stronger. This area will be closer to the mid level shortwave and
upper jet to the north, and deep layer wind shear increases enough
to suggest at least some convective organization is likely. While
not as high as areas further south, instability should still
become sufficient for convection along and south of the warm front
stretching across the region. Overall think convection will be
progressive in nature, although some brief backbuilding/training
is a possibility along/near the warm front. Overall this setup,
combined with PWATs above average, suggests the potential for some
heavy rainfall totals locally exceeding 3", and thus will carry a
Marginal risk.


...Mid Atlantic...

Another active day expected across portions of WV, into southwest
VA, and northern NC. A backdoor front will likely act as a focus
for convective development this afternoon/evening across this
area. PWATs, while not extreme, will remain solidly above normal,
and thus locally heavy rates are possible. Convection may first
develop across portions of eastern KY/TN into southern WV,
southwest VA, and northwest NC...where differential heating in the
terrain may act as an additional trigger for storms on top of the
boundary. Slightly stronger southerly flow across this region
should result in storms drifting off to the north. Instability is
high enough that will probably have some brief cell mergers along
outflows, and also may see some brief training at cell initiation
on/near the terrain. Thus, while in general storms will both be
moving enough and unorganized enough to prevent widespread heavy
totals, likely to see some localized heavy amounts. And given the
saturated conditions, these isolated heavier totals will result in
at least a localized flash flood risk today. Contemplated a Slight
risk given the saturated conditions, but opted to just go with a
marginal for now given the expected unorganized nature of
convection and some chance the higher amounts end up west of the
most saturated soil. Will however need to closely monitor through
the day, as do anticipate at least localized flash flood issues
will arise.

Further east storms will fire along/near the stationary boundary
near the NC/VA border. Instability is forecast to be even higher
here, and mean flow is westerly, parallel to the boundary. Thus
even though shear is weak and suggestive of pulse convection, a
bit more concerned about heavier rains here. Think convection may
try to organize along the boundary, with some regeneration along
outflows allowing the storms to slowly drift south. Thus seems
like a pretty good chance we will see at least localized 3"+
amounts from near the VA/NC border south into northeast NC. These
areas have not been quite as wet as locations further northwest,
so could likely handle a bit more rain before flash flooding
becomes a concern. Nonetheless, the setup suggests at least a
localized flash flood risk is probable and a Marginal risk was
issued.


...Florida and Southeast...

A weak vort and low is currently moving into the Florida panhandle
this morning. This system will bring locally heavy rains as it
moves northward. The 0z high res models are pretty much unanimous
in depicting some very heavy totals in narrow bands with this
feature. In general, think these are probably overdone given the
overall lack of instability. However, locally heavy rains still
seem probable. As the low/wave drifts northward, it should act as
a focus for diurnally driven convection into portions of southeast
AL and southwest GA. Again, instability will probably be a
limiting factor, although still likely to see some locally heavy
convective cores. Again think the high res models are probably
overdone with amounts, but localized totals exceeding 3" seems
likely, with at least a localized flash flood threat given PWATs
increasing to above the climatological 90th percentile. Higher FFG
and the question of instability keeps the threat here at Marginal
for now, but will need to monitor through the day. Should
instability become high enough, the setup does have the potential
to produce a more organized flash flood threat and warrant a
Slight risk.

Chenard



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