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FOUS30 KWBC 252013
QPFERD

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

Day 1
Valid 16Z Thu Jul 25 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.
Warming at 500 hPa should allow for some downtown across AZ today
when compared to previous days.  Weak impulses will provide
enhanced lift beyond the usual diurnal destabilization.  Light
mean flow will persist which will continue to lead to slow storm
motions and increased risk for excessive rain.  RAP guidance shows
a 700 hPa convergence zone across central NV which, along with
portions of the Sierra Nevada, could act as foci for
convergence/lift.  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.

Another slight risk area is over the Sangre De Cristo Mountains of
northeast NM and southeast CO.  Recent diurnal showers and storms
have occurred in the mountains and both the NAM/GFS focus 700 mb
vertical motions underneath 300 mb divergence maxima in this
region centered on 00z Fri within the 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...
A front and outflow boundary could act as foci for convection
today.  The mesoscale guidance is scatter shot, with a general
signal for heavy rain in northern MN and a less obvious signal
near the IA/SD border.  Temperatures at 700 hPa do not appear to
be prohibitive, generally in the 6-9C range.  Low-level
convergence is forecast to broaden and increase with time, with
the low level boundaries moving least in IA/SD.  Precipitable
water values are sufficient for thunderstorms with potential heavy
rainfall, with 1.25-1.75" forecast.  A shortwave moving across
central SD is allowing for an area of 25 kts effective bulk shear
which could organize storms.  Hourly rain totals to 2" should be
possible.  Due to the uncertainty in the guidance presently and
the two week precipitation anomalies in the area being generally
below average, yet the favorable ingredients, thought it prudent
to introduce a marginal risk of excessive rainfall for the region.


...FL peninsula to far southeast GA...
A stationary boundary remains draped across northern Florida which
will be the focus for periodic shower/thunderstorm activity
through Thursday night. An area of 2+ inch precipitable water will
exist across this region in addition to pockets of instability
which maximize in the afternoon hours.  With the upper level jet
streak just to the north, the right entrance region will assist
with large scale forcing along with mid-level impulses moving atop
the surface boundary, an abnormal situation for July.  Therefore,
anticipate convection to move across the northern and central FL
peninsula and storms with higher rain rates focusing near the East
Coast where sea breeze boundaries/outflow boundaries collide and
near the front itself.  Once activity initiates across west
central FL in the morning, low to mid level westerly flow promotes
movement to the eastern peninsula.  A slight risk was not issued
due to their high flash flood guidance -- rainfall during the past
couple weeks has been a bit below normal in the eastern peninsula.

Roth/Petersen



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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