FOUS30 KWBC 170816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Mon Jun 17 2024

Day 1
Valid 12Z Mon Jun 17 2024 - 12Z Tue Jun 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE 
NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, ALONG WITH PARTS OF 
THE CENTRAL AND WESTERN GULF COAST...

...Summary...

Per collaboration with the WFOs, notable adjustments to the 
inherited ERO include dropping the Moderate Risk, which had covered 
much of central MN and western WI in yesterday's Day 2 ERO. There is 
just too much spread in the guidance, including the most recent 
CAMs, to support more than a Slight Risk. The transitory nature of 
the warm front lifting north has a lot to do with this, limiting the 
threat of persistent frontal (west to east) training. Otherwise, 
have expanded the Marginal Risk areas across the Lower MS Valley and 
over the Appalachians, Ohio Valley, and parts of the Northeast based 
on the latest guidance trends.

...Eastern Portions of the Northern Plains into the Upper Midwest...

Stationary surface front across the Central Plains early this 
morning will lift north as a warm front today and tonight as the 
longwave trough drops into the Western U.S., while the broad upper 
ridge builds across the eastern half of the CONUS. Elevated 
convection early in the period along/north of the eventual warm 
front will lift gradually northward today, with a higher likelihood 
of least some cell training through early this morning given the 
uptick in low-level southerly flow (nocturnal LLJ). During the day 
the areas of heavy, potentially excessive rainfall will become more 
spotty with the daytime (differential) heating and weakening low-
level inflow. By tonight, the upper flow will become more 
meridional, with the upper level jet axis more SSW-NNE oriented vs. 
W-E early today. This will shift and confine the best upper level 
forcing (upper divergence and low-level frontogenesis) farther W-NW, 
closer to the surface low track and associated cold front. So while 
the broad Slight Risk encompasses the areas of heavy rainfall today 
along and north of the warm front, the focus for heavy rainfall 
later tonight will shift to the eastern Dakotas and NW-NC Minnesota, 
within the region of aforementioned favorable forcing. All totaled, 
the anomalous deep-layer moisture (TPW anomalies ~2 standard 
deviations above normal) along with other favorable thermodynamic 
and kinematic parameters (MUCAPEs increasing to 1500-2500 J/Kg) will 
favor 1.5 to 2.0+ inch/hr rainfall rates underneath the strongest 
cells and/or where the convection trains. 

...Western to Central Gulf Coast...

A plume of impressive Caribbean and Gulf moisture characterized by 
PWATs as high as 2.25 inches is still expected to move into the 
central Gulf Coast, but forcing for heavy rain on the western 
periphery is quite subtle/transient, with small-scale convectively-
aided mid-level vortices within the COL or deformation zone riding 
northward on the western edge of the ridge. The Slight Risk 
continues to be aligned along the coastal regions, where the 
expectation for forcing is that local Gulf breezes, outflow 
boundaries, and cold pools will provide localized forcing for heavy
convection. However given the low 0-6km bulk shear (aob 20 kts), 
expect predominately pulse-type convection which would become 
outflow dominated rather quickly. Still, given the plentiful 
moisture and instability still available, any storms that form will 
be capable of heavy rainfall rates to the tune of 2 to 3 inches per 
hour, which could still cause widely scattered instances of flash 
flooding. This would especially be the case should these heavy 
rainfall rates occur over an urban or flood sensitive area. Within 
the Slight Risk area, most of the high-res CAMs (including the ARWs 
and 06Z HRRR) indicate isolated pockets of 5.00+ inches of 24hr QPF. 

...Montana and Central Idaho...

The comma-head region of a developing low over Wyoming will impact 
much of Montana and Central Idaho throughout the Day 1/Monday-Monday 
night period. The low will form in the divergence region between 2 
different jet streaks...one to the south over Wyoming, and a second 
over southern Saskatchewan. Unseasonably cold air over western MT 
and ID will clash with moisture tracking westward to the north of 
the low from the excessive rainfall over Minnesota to support the 
rain (and snow at higher elevations) across the region. Upslope flow 
may enhance total precipitation in the mountains in and around Great 
Falls, MT. Cooling from the upslope should support a good amount of 
that precipitation falling as snow at the higher elevations, which 
should cut any resultant flash flooding at the lower elevations to 
just isolated instances (due to lack of runoff from the higher 
elevations). While the amounts of up to an inch of liquid equivalent 
is a wetter rainfall event for this area, they should generally stay 
below FFGs, keeping any flash flooding confined to flood-prone 
areas. No major changes were made, with the portion of the Marginal 
in the mountains of western MT and ID emphasizing isolated valley 
flash flooding, as snow is expected at the higher elevations.

...Lower Great Lakes-Ohio Valley to the Appalachians and Parts of 
the Northeast...

Mainly pulse-variety convection (with 0-6km bulk shear aob 20 kts) 
along the western periphery of the expanding upper ridge may lead to 
localized instances of flash flooding across this area today. This 
as MLCAPE values climb between at least 1000-2000 J/Kg by the 
afternoon, while at the same time TPWs reach ~1.75". High-res CAMs 
show spotty 1.5-2.0+ inch/hr rainfall rates during peak diurnal 
heating this afternoon, then continuing (though becoming more 
isolated) into the evening hours. Most of the CAMs show isolated QPF 
totals of 2-4+ inches, which despite the relatively dry soils 
initially in some areas (relatively high FFGs), could nevertheless 
lead to localized runoff issues, especially over urban areas or more 
sloped terrain. 

Hurley

Day 2
Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024

...THERE'S A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE 
UPPER TEXAS COAST...

...Central Plains...

A cold front pushing east across the northern Plains is expected to
stall out over the central Plains Tuesday and Tuesday night.
Surface high pressure over Montana will dive southeastward into the
Dakotas Tuesday night. The high will provide a north to
northeasterly flow of cooler and drier air into the front.
Northeasterly flow at 850 mb will increase to 30-40 kt Tuesday 
night. Meanwhile...broad southerly flow of deep Gulf moisture will
stream northward across all of the southern Plains and into the
central Plains and the front. The southerly LLJ will peak at 45-55
kts across southern Kansas Tuesday evening. The two highly 
contrasting air masses will slam into each other at the front into
north central Kansas and south central/southeast Nebraska. The 
storms will track northeastward with the deep moisture and 
convergence into northeastern Nebraska...an area that has been very
hard hit with rain in recent days, resulting in very low FFGs. 
Thus, despite the lesser convergence over northeast Nebraska, the 
risk is higher due to more favorable antecedent conditions. Corfidi
Vectors south of the front are out of the north at 5-10 kts. This 
will strongly favor backbuilding and training storms since they 
will be antiparallel (opposite) the prevailing south to 
southwesterly flow of moisture into the front. The two air masses 
colliding head on into each other near the KS/NE border will 
prevent much if any movement of the front...allowing the storms to 
repeatedly impact the same areas as plentiful atmospheric moisture 
replaces that lost from rainfall.

Further north, rainfall across northern MN will be ongoing through
the morning from the overnight rainfall event. While some
convective enhancement is expected due to an abundance of moisture
availability for the convection, as PWATs will be around 1.75
inches. The warm front the storms will be tracking along will be
moving north quickly enough that any flash flooding should continue
with the rain after 12Z Tuesday.  

...Upper Texas Coast...

No significant changes were made to the ERO Risk areas. A plume of
deep tropical moisture originating in the Caribbean will track
northwest across the Gulf of Mexico with a developing tropical low
in the western Gulf. The southeasterly flow around the low will
begin to spread convection with deep tropical moisture and
potential for high rainfall rates into the immediate coast through
the day Tuesday. However, the better forcing and thus, more 
widespread heavy rainfall likely holds off at the coast until 
Tuesday night. There has been a small westward shift, as has been 
common with this rainfall event. Thus, a bit less storm total 
rainfall is expected into southwest Louisiana...with a bit more now
expected into the middle and lower Texas coasts. This should 
overall limit the flooding potential a bit as southern Louisiana 
has been much wetter in recent weeks than south Texas. With 
continued westward shifts in the guidance it's likely the Moderate 
Risk area can be shifted accordingly out of Louisiana and perhaps 
expanded a little along the Texas Gulf Coast. Regardless...most of 
the heavy rainfall event in Texas will be on Day 3/Wednesday except
for the upper Texas coast.

Wegman
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