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FOUS30 KWBC 260101
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Thu Jul 25 2019

Day 1
Valid 01Z Fri Jul 26 2019 - 12Z Fri Jul 26 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NEVADA, COLORADO, & NEW MEXICO...

...Southwest...
Mid/low-level monsoonal flow will persist across the southwest
CONUS around a high moving southwest from the Four Corners.
Scattered convection, with some cells producing brief but intense
rainfall rates/amounts, will gradually be tapering off later this
evening/early overnight hours given the loss of solar heating. RAP
guidance shows a 700 hPa quasi-stationary convergence zone across
central NV which, along with portions of the Sierra Nevada, which
should act to provide focus for on-going and any new convergence
for a few hours.  Given the moisture anomaly of 2 to 3 standard
deviations over central Nevada with most of the state forecast to
have precipitable water values < 1" and an existing flash flood,
the Slight Risk area was maintained with only minor changes to the
boundaries to better fit early evening radar imagery.

Another Slight Risk area was also maintained with little change
from earlier outlooks over the Sangre De Cristo Mountains of
northeast New Mexico and southeast Colorado.  Diurnal convection
was probably nearing the peak of coverage and associated rainfall
rates...and were mainly focused over/near the higher terrain.
Both the NAM/GFS focus 700 mb vertical motions underneath 300 mb
divergence maxima in this region for a few more hours this evening
within a region of light northwest flow.  Low-mid level lapse
rates average near 7-8 deg C/KM, so lift in terrain should result
in formation of showers/storms this afternoon/evening.  Isolated
pockets of heavier rain are possible, with flooding concerns as a
result, in burn scar areas and areas with low flash flood guidance
in the Front Range.


...Northern Plains/Upper Midwest...
Trimmed the southern portion of the Marginal Risk area where the
rainfall rates have been fairly modest and where the higher
resolution guidance has only been showing low percentages of
rainfall rates at or above 0.5 inches.  SPC Mesoscale Analysis
plots of the RAP showed favorable instability all the way from the
international border soutwestward into the eastern Dakotas...and
Precipitable Water values were at or above 1.5 inches.  It appears
that the lack of well defined low level convergence and by
marginal mid-level height falls were working to keep rainfall
rates in check despite the late afternoon instability.  The 18Z
HREF was not indicating any probabilities of 1 hour rainfall rates
exceeding 1 inch in this area.  Farther north, we opted to
maintain a Marginal Risk area closer to International border given
the presence of mid-level height falls on the order of 30-40
meters and Precipitable Water values in the 1.6 to 1.8 inch range.
 Unlike areas to the south, the 18Z run of the HREF did show 40 km
neighborhood probabilities of 1 hour rainfall exceeding an inch
briefly reaching 15 to 20 percent in the 05Z to 08Z time frame.
Thinking that cells should be moving fast enough and steadily
enough to temper the risk of ponding or flash flooding.

Bann



Day 2
Valid 12Z Fri Jul 26 2019 - 12Z Sat Jul 27 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
CENTRAL AND SOUTHERN ROCKIES AND THE CENTRAL INTERMOUNTAIN WEST...

...Central and Southern Rockies and Intermountain West...

A shortwave trough moves east from California into Intermountain
West Friday. As a result, the deep high centered over the four
corners region will shift south and west in time. This will send
further mid-level impulses around the ridge and help promote
widespread shower/thunderstorm development from over NV/UT. In
addition, typical afternoon convective development will occur on
the CO/NM Rockies. With monsoonal flow continuing, expect
precipitable water values to climb to 1.0 to 1.25 inches, which is
2 to 3 standard deviations above the mean. This combined with
moderate instability will lead to periods of heavy rain. Rain
rates look to approach an inch/hour in the heaviest activity.
There is a training threat with light mean westerly flow on the
north side of the high. A focused risk area should become more
apparent with HREF guidance and a Slight Risk is possible.

Jackson

Day 3
Valid 12Z Sat Jul 27 2019 - 12Z Sun Jul 28 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER WEST-CENTRAL
SOUTH DAKOTA...

...Northern Great Plains...
The shortwave trough rounding the high over the Desert Southwest
will push east from the northern Rockies to the Dakotas Saturday
through Saturday night. The low level jet up the Plains Friday
night through Saturday will allow pooling of moisture over South
Dakota (where surface low pressure is expected to track) with
precipitable water around 1.75 inches (2 to 2.5 standard
deviations above normal) expected by Saturday evening. Instability
along and north of a warm front associated with the low will also
pool, which along with light mean westerly flow puts together the
main ingredients for organized, slow moving, and heavy
thunderstorms. Given the flash flood guidance is low over
west-central SD (generally 1 to 2 inches in three hours), decided
to raise a Slight Risk for the Black Hills east through central
SD. This area will be refined in the coming days as the risk area
can be focused.


...Intermountain west/Rockies...
Mid-level impulses riding the ridge over the Desert SW will
interact with the both higher terrain and existing surface
boundaries in the Intermountain West/Rockies and enhance diurnal
convection across this area. With lift, monsoonal moisture and
instability, expect convection to develop during the afternoon
across the higher terrain in WY/UT, the Rockies of CO/NM, and even
the mountains of southwestern NM into AZ. Expanded the Marginal
Risk to cover higher terrain of these states in one outlook area.
Given the diurnal nature, the Marginal Risk may be sufficient, but
given the low mean flow, localized areas may warrant a Slight Risk
in future forecasts.


Jackson


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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