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Day 2 Outlook >
WPC Day 1 Excessive Rainfall Outlook Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 1549 UTC Tue May 30, 2023
Valid: 16 UTC May 30, 2023 - 12 UTC May 31, 2023
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM EDT Tue May 30 2023
Day 1
Valid 16Z Tue May 30 2023 - 12Z Wed May 31 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE WEST, MIDWEST, AND PLAINS...
...Northern CA northeast across ID and southwestern
MT/northwestern WY...
A Mid-upper low off the Central CA Coast this morning will slowly
push inland and wobble through SoCal this evening. Favorable
upper-level diffluence north of the compact mid-upper low, south
of the digging northern stream upper trough/height falls across
the Pacific NW, will generate favorable broad-scale forcing across
the outlook area this afternoon. PWATs are expected to climb to
between 0.75-1.00", which is ~1.5 standard deviations above normal
for the end of May (and above the 90th percentile), which is
sufficient for heavy rain concerns at elevation. Combination of
upper forcing, moisture, weak flow aloft, along with increased
daytime instability (mixed layer CAPEs peaking between 500-1000
J/Kg) will allow for scattered to more numerous showers/storms
within the outlook area -- a few of which will be capable of
hourly or sub-hourly rainfall rates of 0.50-1.00" which is around
FFG values. 12Z HREF neighborhood probs for exceeding 1"/hr rates
and/or 1/3-hr FFG values max out around 40%, supporting at least a
Marginal Risk outline. Lack of focus for convection (aside from
terrain) and dependencies on subsequent convection-induced
mesoscale boundaries precludes a focused Slight Risk contour.
Generally, a localized flash flood risk, especially over
urban/poor drainage areas and any recent burn scars into northeast
CA and southern OR, have a locally higher risk.
...Northern and Central Plains into Midwest...
A pair of mid/upper level shortwave troughs will pivot across
eastern portions of the Northern Plains and across the far
northern MS Valley today. Weak/modest upper divergence combined
with surface lift near and in advance of a frontal boundary will
support a heavy rainfall threat with afternoon and early evening
convection. This largely covers a similar area to the SPC area as
well. PWATs look to peak around 1.3" (near the 75th percentile,
per TOP sounding climatology), with SB CAPE rising to as high as
3000 J/Kg into the outlook area. Low-level inflow and effective
bulk shear appear sufficient to organize convection that forms,
especially initially along the CO/KS border and moving east.
Southern extent has potential for a more appreciable excessive
rainfall threat (OK/TX Panhandles, where FFG values are a bit
lower) depending on the forward speed and development of the
southern end of convection. 12Z HREF neighborhood exceedance
probabilities continue to suggest the potential for locally
extreme amounts (3-5"+), but it is too difficult to pinpoint a
specific area within this broad region (as indicated by a lack of
consistent HREF EAS neighborhood probabilities for 1" exceedance).
Did adjust the area slightly over southwestern MO and the TX
Panhandle to account for some 12Z CAM guidance that showed
potential for convection farther south later this
evening/overnight.
...Southeast OK into Northeast TX and Ark-La-Tex...
Precipitable water values of 1.5"+ and SB CAPE expected to
increase to 1000-3000 J/kg support the Marginal Risk area near the
Ark-La-Tex this afternoon. Low-level inflow appears weak, only on
the order of 10-15 kts, but the mean 850-300 hPa winds are
virtually null (which suggests minimal if not chaotic storm motion
and better rainfall efficiency than average from storms that do
form and maintain). Effective bulk shear may be as high as 25 kts
on the western edge of the MRGL risk area, which may help to
organize storms (and would potentially organize a complex, should
upscale growth be sufficient). Even so, FFG values are quite high
in this area so any excessive rainfall concerns should remain
isolated.
Fracasso/Churchill/Roth
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
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