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FOUS30 KWBC 210120
QPFERD
Excessive Rainfall Discussion...Amended
NWS Weather Prediction Center College Park MD
918 PM EDT Mon Aug 20 2018
Day 1
Valid 01Z Tue Aug 21 2018 - 12Z Tue Aug 21 2018
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL NEAR AND WEST OF
MADISON WI...
...Central to northern Iowa..southeast Minnesota...southern
Wisconsin into northern Illinois...
The strong closed low moving across MO/IA at this time will
steadily east northeast push through the mid Mississippi Valley
region and toward the southwest Lower Lakes region tonight/early
Tuesday. An axis of heavy precipitation is associated with what
will likely remain a well defined comma head/deformation
precipitation band to the north of the track of this mid to upper
level closed low. Precipitation areas in this comma
head/deformation have been slow moving...pivoting this evening
across southern Wisconsin before an expected shift into southwest
Michigan later tonight. Radar estimates west of Madison,
Wisconsin are near 8", which has led to the upgrade to a moderate
risk. A combination of a mesocyclone, cell mergers, and minimal
storm complex motion has led to these totals. Widespread areal
average totals in the 1-3" range within the slight risk area are expected...with isolated heavier totals possibly reaching 10-12"
near and west of Madison, Wisconsin within the moderate risk area
before the threat shifts eastward in a few hours.
...Middle to Lower Mississippi Valley...Lower Ohio Valley...Lower
Tennessee Valley
To the south of the expected well defined comma head/deformation
precipitation band with the strong moving into the mid Mississippi
Valley today...additional pre-frontal/frontal convection is moving
swiftly through Lower Mississippi and Lower Ohio Valleys in a
region of defined upper difluence and boundary layer convergence.
This activity is expected to then push eastward farther into the
Ohio and lower Tennessee Valleys tonight. While it should be
fairly progressive---but with unstable conditions ahead of this
front...mu-cape values 1000-2000+ j/kg and PW values above
2"...locally heavy rainfall amounts possible.
...Southern Mid-Atlantic...Southern Appalachians into the Central
Gulf Coastal region...
Storms continue to fire across NC near and north of a mesoscale
boundary, on the backside of an upper level low. Until convection
fades within the next several hours, hourly rain totals to 2.5"
are possible within this saturated airmass. Pockets of the region
have received 3-5" of rain over the past couple days, which merits
the continuation of a risk area. Spotty amounts have kept the risk
marginal.
Roth/Pereira/Oravec
Day 2
Valid 12Z Tue Aug 21 2018 - 12Z Wed Aug 22 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL NORTH FROM THE
CENTRAL APPALACHIANS AND IN ARIZONA...
...Eastern Great Lakes to Mid-Atlantic Region..
A shortwave trough will be drawn northeast across the Great Lakes
to the base of a larger-scale trough extending from Hudson Bay
Tuesday. Southerly flow, deep moisture and the formation of a
mid/upper level jet streak will set the stage for afternoon and
night time storms capable of producing heavy rainfall south from
the eastern Great Lakes. A Slight Risk was maintained over
portions of the Eastern Great Lakes to the central Appalachians
given the Precipitable Water Values of 1.75 to 2.00 inches (2 to
2.5 standardized anomalies above climatology) expected. Eastern
portions of this area have had above normal rainfall in the past
month.
Farther south over the southern Appalachians and VA/NC Tidewater
forcing and instability remain limited, so a Marginal Risk was
maintained despite well above normal moisture.
...Desert Southwest....
A plume of deep moisture continues to advance north on return flow
west of the high centered over NM/TX Tuesday. Precipitable water
values of 1.75 inches drawn from southwest Arizona will cross the
higher terrain of central AZ. Precipitable Water values of an inch
or more (generally two standard deviations above normal in this
high terrain area) will reach the southern Wasatch by late Tuesday
afternoon. This will support scattered late day and evening
convection that will be capable of producing locally heavy
rainfall across the four corners region. Maintained a Slight Risk
over portions of Arizona where the moisture will be deepest and
where shortwave energy is expected to rotate around the periphery
of a mid/upper level high to the east. Will have to continue to
monitor smaller scale features and timing for the most at risk
areas.
...South-Central High Plains...
A shortwave riding zonal flow south of the northwestern CONUS
trough will encounter the nocturnal low level jet over the Plains
Tuesday night. 1.5 inch PW (1 to 1.5 standard deviations above
normal) will allow thunderstorms to organize (most 12Z guidance
suggest two tracks across KS and the Panhandles). A broad Marginal
Risk continues to cover both of these areas.
Jackson
Day 3
Valid 12Z Wed Aug 22 2018 - 12Z Thu Aug 23 2018
...MARGINAL RISK FOR EXCESSIVE OVER NEW ENGLAND...CENTRAL
PLAINS...AND THE SOUTHWESTERN U.S....
...Northern New England...
Maintained a Marginal Risk over northern New England as moist flow
from the Atlantic Ocean gets drawn inland by a developing low
pressure moving into Canada from the eastern Great Lakes
Wednesday. Plenty of confluent flow at the low levels being drawn
into the hilly terrain of northern New England is expected.
Instability looks quite limited per the 12Z GFS/ECMWF, so the
Marginal Risk was maintained for now.
...Southwest U.S....
Moisture being drawn northward across AZ shifts east a bit
centering on the four corners. Ample moisture (generally 1.5
standard deviations above normal) and instability will allow heavy thunderstorms. However, weak forcing suggests widely scattered
heavy rain with the Marginal Risk maintained for now.
...Central Plains...
Convection forming over the Central/Southern Plains early
Wednesday morning will taper off at the time of convective minimum
over parts of Kansas or Oklahoma, followed by an additional round
of convection forming during the latter portion of Wednesday and
the early-hours of Thursday morning as 850 mb winds ramp up in a
low-level jet of 25 to 35 knots. 12Z guidance converged a bit on a
QPF max just north of the KS/OK border which has been fairly wet
over the past week. Maintained a Marginal Risk.
Jackson
Day 1 threat area:
http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area:
http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area:
http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt
$$
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