Quantitative Precipitation Forecast
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FOUS30 KWBC 131555

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Mon Jul 13 2020

Day 1
Valid 16Z Mon Jul 13 2020 - 12Z Tue Jul 14 2020


...Northern Great Plains to Lake Superior...
Thunderstorms will initiate this afternoon, where a strong
southerly low-level jet (40-45 kts at 850 mb) and associated
theta-e advection intersects the northern cusp of a plume of
strong instability, most likely in E ND and NW MN. Although
precipitable water values will exceed 150 percent of normal in the
same area (near 1.75" in a pre-convective environment), supporting
high instantaneous rain rates, most model guidance indicates
steady eastward progression of the convective cluster that
develops. This is evident in the HREF mean 3-hr rainfall
progression, which shows very little overlap between the rainfall
footprint in successive 3-hr periods. Therefore, have maintained a
broad Marginal Risk was maintained across this region from
yesterday`s day 2 ERO. However, we have also included a Slight
Risk area across much of MN, northeast ND, southeast SD, and far
northwest NE to northern IA. Over the northern portions of the
Slight Risk area, the favorable deep-layer forcing (upper
divergence) is expected to be a bit more robust and last a little
while longer into the evening and early overnight hours.  In
northern areas near the MN/ND border, the National Water Model
showed a rapid response to the expected heavy rainfall which helps
bolster the idea of a Slight Risk area.  Farther south-southwest
across southern MN into southeast SD and northeast NE, outflow
boundaries will likely generate a more WSW-ENE "effective" front
south of the main sw to ne oriented cold front. The increasing
west to east drape will favor an enhanced potential for cell
training Monday night -- supported by the veering (southwesterly)
low-level jet aligning parallel to (while exceeding the magnitude
of) the mean 850-300 mb wind. Within these areas, the models show
Corfidi vectors weakening to 10 kts or less, while becoming more
easterly with time after 06Z Tuesday (supportive of increased
upwind propagation and cell training).

Two primary rounds of convection will bring a threat of flash
flooding to the Northeast: (1) very shortly ahead of an advancing
cold front from the Connecticut River Valley through southern New
Hampshire and to the southern coast of Maine, and (2) late Monday
Night from far northern New York into Vermont with a mid-upper
level low. A Slight Risk was introduced in yesterday`s day 2 ERO
for the region to account for both rounds.

With the near term convection, the hi-res models is in good
agreement in showing a significant uptick in the 16-19Z time frame
as instability maximizes and MLCAPE likely reaches 2000 j/kg in
some areas -- currently 500-1000 J/kg across NY and much of New
England.  A surface wave near southern New Hampshire appears to be
slowly backup up a front into southeast ME at this time.
Simulated reflectivity shows multicell clusters and lines forming,
with relatively fast northeastward storm motion. However, hi-res
models also indicate the development will be taking place in a
SW-NE oriented corridor from CT into C MA and S NH along the
trailing front, so training of convective bands may occur over a
couple hours at any given location. The combination of the strong
instability and precipitable water values in excess of 1.5 inches
should yield high instantaneous rain rates. With the potential for
some narrow swaths of heavy 1-2hr rainfall due to training
convection (supported by the uptick in 00Z HREF 40km neighborhood
probabilities), along with a relatively high coverage of urban
areas in the region, there does appear to be a corridor with a
higher flash flood threat, warranting an upgrade to Slight Risk.

On Monday Night, as the first round of convection wanes and pushes
out over the Atlantic, an upper level low will shift from eastern
Ontario to near the Champlain Valley. The cold air aloft and
steepening mid-level lapse rates should be sufficient to offset
the loss of insolation and maintain available CAPE around 1000
j/kg. Furthermore, the deep layer mean wind speeds are expected to
decrease from around 15-20 knots at 18Z Monday, to around 5 knots
by 06Z Tuesday. This should support the development of slow-moving
thunderstorms as the DPVA  maximizes around 06Z. HREF neighborhood
probabilities of 40+ dBZ reflectivity increase to over 70 percent
over VT after 06Z. Given the potential for very slow-moving
convection, with relatively widespread coverage, and rain rates of
1-2 inches per hour, the Slight Risk of excessive rainfall will
likely persist overnight from northern NY into VT-NH and southern

...Eastern North Carolina and southeast Virginia...
A Marginal Risk was maintained along the Atlantic Coast over
eastern North Carolina and southeast Virginia, slightly expanded
in this update to include the ideas from the 12z HREF.  Although
antecedent conditions are not particularly favorable (outside of
southeast NC and northeast SC which show two week precipitation of
100-300% of average), most of the area has experienced below
average precipitation but the high-res models continue to show
localized rainfall maxima in excess of 3 inches on a time scale of
generally 3 hours or less.  In fact, the 00Z HREF 40km
neighborhood probabilities of 2 inch per hour rainfall rates reach
as high as 50 percent in the Virginia Beach to northeast NC areas
this afternoon and non-zero values extend over much of the rest of
the Marginal Risk area. The combination of strong instability and
PW values over 1.75 inches will support such heavy rain rates, and
flash flooding would be most likely if the heavy rain rates were
to occur over urban areas.

...Lower Mississippi Valley to the Central Gulf Coast...
Heavy rainfall potential remains across the region this afternoon
and evening ahead of the approaching MCV currently over KS/OK
early this morning, should convection fully break the mid-level
capping inversion.  There are favorable factors for heavy to
locally excessive rainfall rates, with PWs in the pre-convective
environment around 2.00" and robust deep layer instability toward
peak diurnal heating (mixed-layer CAPES currently 3000-4000 j/kg
with reducing CIN which should be able to broach 5000 J/kg later
today and might fully erode CIN).  The big issue outside of
southern LA is that 700 hPa temperatures are 12C+ as the center of
a high at that level is nearby across the Piney Woods of East TX.
Despite the expected increase in convective coverage toward
midday, especially across Southern LA, 0-6km bulk shear values
have fallen to just under 25 kts which would likely lead to
short-lived intense rainfall rates given the weak flow the
thunderstorms may be more pulse in nature/not well organized
despite significant upward growth where the mid-level cap/lid can
be broken due to the high CAPE values anticipated.  Isolated cores
of heavy rainfall will be capable of producing between 2 to 3
inches of rainfall within an hour prior to collapsing, and as such
will likely lead to localized runoff issues.  Cell mergers could
allow rain totals to locally exceed the 4-5" in three hour flash
flood guidance in a potentially volatile environment, which allows
for a continuation of the Marginal Risk area.


Day 2
Valid 12Z Tue Jul 14 2020 - 12Z Wed Jul 15 2020


...South-Central High Plains to Western Great Lakes...
A cold front will continue to push southeast across the central
Plains (particularly as lee-side surface cyclogenesis shifts south
over the central High Plains) and east into the Great Lakes
Tuesday. This sets up a SW-NE oriented low-level front with broad
southwesterly flow roughly parallel for much of the period. This
general configuration could favor training convection, and the
preferred 00Z NAM/GFS/ECMWF indicate a swath of heavy rainfall
with maximum values over 2 inches from southeast MN, over the
northwestern 2/3 of WI, into the UP of MI. Consideration was again
given to an upgrade to Slight Risk, but there remains too much
uncertainty as to the position of any convection at the start of
the period from the previous day, associated cloud debris, and
impacts on the available instability in the warm sector. For
instance, a forward propagating MCS with a stronger cold pool
tonight could reconfigure mesoscale boundaries in a drastic way
and lead to very different placement of the heaviest rainfall
amounts. Of note is the new FV3SAR CAM which is more progressive
and farther east than other guidance. Therefore, the large
Marginal Risk was maintained for now.

Back over the central Plains low level flow backs to southerly,
sending a surge of Gulf moisture into central KS and around the
south-shifting lee-side low over the High Plains. This makes for a
risk for elevated thunderstorms in an environment with PWs of 1.5
inches (1.5 to 2 standard deviations above normal) and low level
flow wrapping around the low counter to the higher level westerly
flow leading to upwind propagation and potentially excessive rain.
Expanded the Marginal Risk to southwest KS and into the OK/TX
Panhandles due to this risk.

...New England...
The Marginal Risk over New England was expanded again, south into
MA. This will be a continuation of the flash flood threat that is
expected to develop late tonight and continue into Tuesday. An
upper level low will progress through New England. Weak flow near
its center, the strong PVA aloft, and moderate instability should
support continued localized heavy rainfall with rain rates
exceeding 1 inch per hour at times. Some consideration was given
to a small Slight Risk upgrade given the expectation for very slow
storm motions, but uncertainty on exact placement led to another
continuation of the Marginal Risk.


Day 3
Valid 12Z Wed Jul 15 2020 - 12Z Thu Jul 16 2020


...Central Plains to Central Great Lakes...
Prefrontal activity is expected to be ongoing over the
east-central Plains (northern MO/southern IA) Wednesday morning
with a plume of 2 inch PWs shifting northeast ahead over the front
over southern WI/northern IL and then across the LP of MI through
Wednesday evening. This moisture is generally 2.5 standard
deviations above normal and will be worthy of a Slight Risk, but
for now the timing uncertainty is great enough to preclude higher
than a Marginal. Furthermore, this swath has been fairly dry over
the past week except for southern WI/far northern IL where the
lowest FFG currently resides (less than 2 inches in 3 hours).
Another limiting factor is strong southwesterly flow ahead of the
front which will keep storm motion progressive and cold pool
generation should keep it propagating into the more unstable air
to the southeast potentially limiting training. Good agreement
among the 00Z GFS and ECMWF for the heaviest QPF axis warrants a
long swath of 1.5 to 2 inch QPF from northern MO into central MI.
This scenario warrants continued close examination.

...Northeast New Mexico...
A cold front and lee-side surface low will shift south into the TX
panhandle Wednesday. Northeasterly post-frontal low level flow
will orographically lift over the southern High Plains and up
against the Sangre de Cristo Mtns across the CO/NM border.
Diurnally driven convection will occur in this above normal
moisture airmass which along with weakening steering flow should
result in cell mergers and some repeating heavy rain over the
southern High Plains. Included a Marginal Risk for northeast NM
into neighboring CO/OK/TX.


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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