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FOUS30 KWBC 212030
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Sun Jul 21 2019

Day 1
Valid 1951Z Sun Jul 21 2019 - 12Z Mon Jul 22 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS INTO THE LOWER MISSOURI VALLEY...IN THE LEE OF
THE CENTRAL TO SOUTHERN ROCKIES AND ACROSS THE UPPER OHIO VALLEY
SOUTH INTO THE MIDDLE TO UPPER TENNESSEE VALLEY...

...16Z update...

...Lower Missouri Valley/Middle Mississippi Valley...
The Slight Risk was extended southeastward to account for the 12Z
hi-res consensus for 2-4 inches (locally higher) of rain from
southeastern Missouri into western Kentucky through Monday
morning. Two low-mid level vorticity maxima/MCVs analyzed (1) over
the Nebraska/Kansas border and (2) west-central Illinois are
expected to reignite with convection late this evening and
potentially link up in between with possible back building along a
cold front forecast to move into the lower Ohio Valley by about
00Z. The 12Z HRRR, experimental HRRR, regional GEM, arw,
nam_conest and nssl (to some extent) support 3-5+ inches across
the Mississippi Valley between 00Z and 12Z tonight.

Otto

...previous discussion follows...

...Central Plains into the lower Missouri Valley...
Shortwave energy pushing eastward into the Central Plains this
morning will help ignite another round of potentially organized
convection along the slow moving frontal boundary lying west
southwest to east northeast from the Central Plains into the Lower
Missouri and Mid Mississippi Valley region.  Low level inflow into
this front expected to be enhanced this afternoon ahead of the
above mentioned shortwave..with pw values rising to 1.5 to 2+
standard deviations above the mean across much of KS...southern NE
into western MO.  This should help support increasing coverage of
convection in the vicinity of this front Sunday afternoon across
south central NE/north central KS...with this activity pushing
southeastward across eastern KS into western MO later Sunday
afternoon into Sunday night.  There is a strong model signal for
heavy precip totals from southeast NE...eastern KS and western MO
with the slight risk drawn in the model consensus heavy precip
axis.  Href neighborhood probabilities for 2"+ totals this period
are quite high across this region...60 to 90 %...45-70% for 3"+
amounts and 15-30% for 5" amounts.

...Lee of the Central to Southern Rockies...
Post frontal east northeasterly flow expected today in the lee of
the Central to Southern Rockies.  This will help initiate
convection from the eastern slopes of the Central to Southern
Rockies Sunday afternoon with cells moving slowly eastward into
central to southern High Plains Sunday night into early Monday.
There continues to be model agreement that convection may become
organized  in the vicinity of southeast CO/northeast NM and the
western portion of the TX/OK panhandle region Sunday night/early
Monday morning.  This region is showing the highest href
neighborhood  probabilities for 2"+...40%+ and 20%+ probabilities
for 3"+ amounts this period.  The previous slight risk area was
expanded slightly to encompass the model spread of heavy precip
values across this area.

...Upper Ohio Valley southwest into the Upper to Middle Tennessee
Valley region....
The broad slight risk area was maintained from the previous
forecast over the Upper Ohio Valley region and expanded farther to
the southwest into the Upper to Middle Tennessee Valley region.
Two areas of mid to upper level height falls expected to enhance
convection across these regions this period.  Northern shortwave
energy is expected to push east southeast along the southern edge
of the westerlies...enhancing large scale uvvs in the axis of
above average  pw values ...1.5 to 2+ standard deviations above
the mean and mu-cape values 2000-3000 j/kg across the Middle to
Upper Ohio Valley region.  Model consensus is for widespread
moderate to heavy totals across this region where href
neighborhood probabilities show 50-70%+ probabilities of 2"+ and
30-50% for 3"+ amounts.  Farther to the southwest...a mid to upper
level shear axis from central to eastern TN into northern AL and
northern MS should enhance convective potential across this region
late morning into this afternoon.  As mentioned above...the slight
risk was expanded farther to the southwest into these regions to
cover the higher href neighborhood probabilities for 2 and 3"+
amounts across regions where the ffg values are relatively low.

...Northern Maine...
The Marginal Risk was adjusted to account for activity forming
over Quebec just ahead of the secondary cold front crossing
northernmost ME. While the airmass over northern ME has been
overturned by earlier storms, another bubble of 500-1000 J/KG of
MLCAPE over southwest Quebec could support low topped convection
capable of hourly rainfall rates near 1.25 inches. Three hour
flash flood guidance values are as low as 1.50 inches over this
area, so a low end flash flood threat remains through the frontal
passage before 22/00z.


Oravec/Hayes



Day 2
Valid 12Z Mon Jul 22 2019 - 12Z Tue Jul 23 2019

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN MID ATLANTIC AND NORTHEAST...

...Lower MS Valley/TN Valley/Appalachians/Mid-Atlantic/Northeast...

2000 UTC Update -- Have expanded the Moderate Risk area slightly,
to encompass more of the Urban Corridor (DC-Balt area), along with
more of northern NJ into far southern NY (including NYC), western
LI, and far southwest CT. This was based largely on the antecedent
soil moisture and/or relatively low short term FFG. The models
continue to advertise a heavy rainfall signal within this area,
with the outlook period now fully captured by the 12Z high-res
windows. Isolated-scattered 3-5+ inch totals were noted per the
guidance, especially the CAMS, ahead of a slow moving frontal
boundary where for a period of time the 850 mb flow is expected to
become nearly parallel and of similar magnitude of the mean
850-300 mb wind. This will lead to an enhanced risk of training
convection, with again the metropolitan areas along with other
locations with wetter soils/lower FFGs most susceptible. For more
on the meteorological setup, please refer to the discussion below.
-Hurley

A cold front will slowly cross the TN Valley and Appalachians
Mountains Monday/Monday night while slowly sagging southeast
through the Mid-Atlantic and Northeast as a deep trough digs
across the Great Lakes. Moisture and instability will pool along
and south of this boundary with PWATs 2 to 2.25 inches (which is
2-2.5 standard deviations above the mean) across portions of the
TN Valley, banking up against the western slopes of the
Appalachians and across the Mid-Atlantic region Monday
afternoon/evening. Pockets of instability over 2000 J/kg was noted
within the model soundings, especially across the Mid-Atlantic
region. The instability gradient seems to bisect PA/NJ which will
likely play a role in convective development and evolution.  Some
concern also remains in terms of destabilization from cloud cover
during the morning into early afternoon across portions of PA.
Within respect to the mid-level, impulses will continue to round
the aforementioned trough helping to promote lift ahead of the
surface front.  An upper level jet axis will pivot south into the
OH Valley positioning the right entrance region across the central
Appalachians into the Mid-Atlantic and Northeast by later in the
period. Plentiful mid/upper level support with such a strong
baroclinic gradient suggests widespread and potentially multiple
rounds of thunderstorms.  The biggest discrepancies among the
models is the timing of the frontal passage.  Regardless, there is
clearly a multi-model signal for excessive rainfall that stretches
from the TN Valley northeast into New England.

The main area of concern with respect to heavy precipitation will
be from the expected pre-frontal convection which will impact
portions of the TN Valley into the central/northern Appalachians.
Areal average precipitation in this area is around 1-2+ inches
with higher values possible thanks to training convection as the
mean wind becomes better aligned with the corfidi vectors. Given
the sensitive soils, especially across portions of the
Mid-Atlantic, and better model signal/ingredients for heavy
rainfall across portions of MD/PA/NJ, introduced a Moderate Risk
to highlight the potential for more widespread flooding concerns.
This upgrade was closely collaborated with local offices,
RLX/PBZ/CTP/LWX/PHI. Hourly rain rates could exceed 2+ inches and
with 3 hourly FFG as low as 1.5 inches in some locations.  Another
area of concern is across portions of the TN Valley where heavy
rainfall is forecast to occur on Day 1 which may reduce FFG values
even more. Have a Slight Risk across this area, extending
northeast. A Marginal Risk stretches as far south as the Lower MS
Valley where they have received quite a bit of rain associated
with Barry but are quick rebounding.

...Southern Rockies...

Easterly flow behind the cold front pushing south into Mexico will
direct moist air into the southern Rockies topography. PWATs 2
standard deviations above normal and some Monday afternoon
instability warrants a Marginal Risk for eastern slopes of the NM
and southern CO Rockies.

Pagano


Day 3
Valid 12Z Tue Jul 23 2019 - 12Z Wed Jul 24 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC INTO THE SOUTHEAST...

...Virginia/Carolinas/Georgia...
2000 UTC Update -- Upgraded a large portion of the Marginal Risk
area to a Slight -- based on the 12Z model trends in depicting a
rather large heavy rainfall footprint ahead of the advancing upper
trough and associated surface cold front. Nudged the northern edge
of the Slight Risk toward the southern edge of the day 2 Slight,
in order to prevent a gap between both days (i.e. same
event/similar meteorological setup straddling 12Z Tue).
Areal-average QPF within the Slight risk areas is considerably
higher compared to previous forecast cycles -- based on a
multi-model blend centered around the NBM and WPC bias-corrected
ensemble QPF. These ensembles indicated a fairly widespread swath
of 1.5-2.5+ inches of rain, which is quite noteworthy considering
the absence of high-res guidance during the period. -Hurley

Previous discussion..

A front will continue to move south and east Tuesday shifting off
the NC coast by early Wednesday as a longwave trough pivots
through the OH/TN Valley.  Moisture and instability will pool
along and just south/east of the front allowing for pre-frontal
convection to likely develop Tuesday afternoon/evening across
eastern portions of VA/Carolinas and Georgia. There is strong
evidence for heavy rainfall as seen by the multi-model signal for
high QPF totals, especially across the piedmont/sand-hills of
VA/NC/SC.  The timing of the front will certainly play a role in
terms of convective initiation and the evolution through late
afternoon/evening.  With precipitable water values climbing above
2 inches and MUCAPE values around 3500 J/kg, the ingredients point
to storms being heavy rainmakers.  Mid-level energy moving atop
the surface front combined with the right entrance region of the
upper level 125kt jet will only act to enhance lift across this
region.  Given this, fully expect a round or two of convection
with the potential for training as noted by the mean flow becoming
better aligned with the corfidi vectors.

Areal average precipitation of 1-2+ inches with locally higher
values can be expected.  Hourly rain rates may also exceed 1.5
inches. However, given very high flash flood guidance across this
region, felt flash flooding could be somewhat limited. Therefore,
introduced a Marginal Risk for excessive rainfall.  If QPF totals
increase and there is better model agreement on timing of the
front and the overall placement of the highest QPF, a Slight Risk
may be warranted, especially across portions of eastern VA/NC,
where soil saturation is slightly higher per the National Water
Model.

...Southwest/Southern Rockies...

With the cold front well south of the area and high pressure
continuing to build in from the north, easterly flow will enhance
upslope convection across the higher terrain.  Low to mid-level
moisture will still be present as noted by precipitable water
values of 1-1.25. Instability will climb to over 1500 J/kg MUCAPE.
 And with slow storm motion just after convective initiation, this
will exacerbate the potential for flash flooding across this
region.  Therefore a Marginal Risk was introduced across portions
of the higher terrain in AZ/NM/CO.

Pagano


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


$$




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