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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 26 2024

Day 1
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...

...Summary...
Per collaboration with WFOs ABR, FSD, MPX, and ARX, have hoisted a
Slight Risk across eastern SD into southern MN, western WI, and a
small portion of northeast IA. Have also added Marginal Risk areas
across the Mid-Upper TX Coast to eastern portions of the Hill
Country, much of the west coast of FL, and across parts of New
England.

...Northern Plains into the Upper Mississippi Valley...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains on Monday, becoming less
amplified with time as it encounters the central U.S. ridge.
Precipitable water ahead of the upper trough and associated surface
cold front area expected to peak between 1.75-2.00 inches over the
outlook areas. Pre-frontal surface low tracking slowly from northeast
NE-southwest SD early Mon to the eastern MN-western WI border by
12Z Tue will coincide with a slower eastward frontal progression
later Monday and especially Monday night. As a result expect
multiple rounds of organized convection across the outlook areas,
with multicellular upscale growth potential along with some
supercells. MLCAPEs of 3000-4000+ J/Kg this afternoon-evening within
the very moist environment will support intense instantaneous
rainfall rates; however, given the relatively weak llvl flow
compared to the mid-upper levels, expect the convection to be
forward propagating at a fairly decent clip through this evening.
This will cut down on the rainfall potential during the daylight
hours. However, given the slow moving surface low and front, again
expect another round of convection Mon night as PWs remain between
1.75-2.00 inches with MUCAPEs still between 1000-2000 J/Kg. The
aggregate of both events will likely enhance the flash flood risk
within the Slight Risk area, given the growing support from the
high-res CAMs.

Elsewhere, the Marginal Risk area remains, though did pull the
southern edge south a bit into much of southern WI (including the
MKE area) towards the max instability gradient.

...Mid-Upper TX Coast to Eastern Portions of the Hill Country...
Well defined low-mid level circulation near the TX Gulf Coast early
this morning will traverse slowly into South-Central TX this
evening and overnight. Instability will remain greatest offshore,
though SBCAPEs near 1000 J/Kg distributed in a tall/skinny profile,
with WBZ levels over 13 kft and PWs of 2.00-2.25 inches, will
produces areas of heavy rain. Given the weak deep-layer flow (0-6km
bulk shear 20kts or less), expect the more intense rainfall rates
to be short lived before the more intense updrafts collapse as the
cells become outflow dominated. Therefore have opted for a Marginal
Risk over this region, with the anticipation that any flash
flooding/short term runoff concerns will be isolated.

...Western FL Peninsula...
The guidance shows a more enhanced ECSB (East Coast Sea Breeze)
given the added synoptic low-level easterly component. This will
focus peak diurnal convective coverage along the west coast of the
FL Peninsula, which is supported by the model QPFs (especially
CAMs). PWs between 2.1-2.3 inches, along with SB CAPEs peaking
between 1500-2500 during the afternoon and early evening will
support intense short term rainfall rates. In fact, the 00Z HREF
40km neighborhood probability of 2"/hr rainfall rates climbs to
40-60% over/near the Tampa Bay area between 20-00Z (while 3"/hr
rain rates climb to around 25%). Per the CAMs, localized totals of
3-5" can be expected, much of which would be within a 3 hour
period. Cannot rule out localized flash flooding as a result,
particularly in urban areas in and around the Tampa Bay area.

...Parts of New England...
Vigorous shortwave energy and compact LFQ forcing pivoting south-
southeast on the western periphery of the upper low will set the
stage for fairly robust deep-layer forcing over a relatively narrow
corridor across parts of New England Today. The mid/upper ridge
impinging from the west is likely causing the uptick in upper level
flow (localized 90-100kt 250mb jet streak) across the Hudson
Valley/eastern NY. Steepening mid level lapse rates along with
MLCAPEs ~1000 J/Kg will help fuel numerous showers/tstms during the
peak heating hours this afternoon, with storm motions south-
southeast. The 00Z HREF depicts high probabilities of >1"/hr rates
between 18-00Z, peaking between 60-70% across eastern
CT/RI/southeast MA. Localized storm totals of 3+ inches may lead to
isolated instances of flash flooding.

...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but anomalously strong steering flow
should help limit residence time to mitigate some of the flash
flooding concern. As mentioned earlier, although even brief heavy
rainfall rates in a short period of time may still lead to flash
flooding, and a Marginal Risk was maintained in the region.

Hurley


Day 2
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHWEST AND UPPER MIDWEST...

...Summary...
Have expanded the Marginal Risk southward across the Upper Midwest,
to account for the potential of locally heavy rainfall before the
frontal passage.

...Upper Midwest-Upper Great Lakes...
Upper shortwave will continue to dampen somewhat by Day 2, which
will weaken the deep-layer Qs forcing. Prior to the surface frontal
passage, PWs will remain between 1.75-2.00 inches (around 2
standard deviations above normal). Instability will be much greater
over the southern portion of the outlook area (down towards the
I-80 corridor, which was a big reason to nudge the ERO Marginal
Risk farther south (matching up with SPC's Day 2 Severe Slight
Risk). Expect the activity to move rather swiftly before dropping
south of the outlook area Tue night. Still appears to be a
localized flash flood risk though, given the weakening deep-layer
forcing and degree of downwind propagation. Therefore, have
continued with the Marginal Risk.

...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon
and evening across portions of the Southwest US as a moisture plume
lingers across the area and winds aloft/steering flow weakens in
response to the a mid level wave moves away over the Northern
Plains as it begins to fill. Model based ingredients suggest the
potential for locally heavy downpours may extend a bit west of the
area shown by the deterministic runs...so made a minor expansion
that way.

Hurley
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