Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
347 AM EDT Sat Sep 14 2019
Valid 12Z Sat Sep 14 2019 - 12Z Sun Sep 15 2019
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE UPPER MIDWEST AND SOUTHWEST...
A warm advection pattern strengthens this period as a low pressure
area moves by to the northwest. Precipitable water values surge
to 1.25-1.5" before the convection breaks out early evening.
Temperatures at 700 hPa imply there is no significant capping
inversion present. Although convection could be slow moving at
first across southern MN, for the most part it should be
progressive to the south-southeast to southeast per convective
propagation vectors and the 1000-500 hPa thickness pattern.
Thought the high resolution 00z NAM (or a southwest adjusted 00z
Canadian regional) had the best concept of what was to come with
convection slowly building up in scale as it marches along and
increasing PWs above 1.75". There are some concerns for
non-traditional cell training where activity along the warm front
aligns as west-southwest inflow at 850 hPa of 25+ knots exists
over a broad area and activity moves southeast to south-southeast.
ML CAPE up to 4000 J/kg is forecast to exist upstream to the west
per the 21z SREF. Hourly rain totals to 2" with local amounts to
4" should be supported in this area. The big problem across MN,
WI, northern IL and portions of northeast IA are the two week
precipitation anomalies, which are 200-600% of average, implying
saturated soils. The three hourly flash flood guidance values
agree, with 1-1.5" in three hours necessary for exceedance. Due
to this, was liberal with the breadth of the marginal and slight
risk areas depicted.
A disturbance aloft moving in from northwest Mexico is forecast to
intensify as it crosses the AZ border at the end of the period by
the available model guidance. Precipitable water values of
1.25-1.5" are expected to move in near and ahead of this feature.
Daytime heating is expected to bring ML CAPE values up to 2000
J/kg during the late morning and afternoon. Inflow at 850/700 hPa
increases with time, to at least 30 knots by the end of the day 1
period (early Sunday morning). However, the mean 850-400 hPa wind
is not forecast to exceed 15 kts, which should set up
precipitation efficiency. Where cells train or merge, hourly rain
totals to 2" should be possible, which would be most problematic
in urban environment and washes. The 00z HREF probabilities of
exceeding 1" an hour, along with the low level flow, suggest
activity from southeast AZ should move west into the valleys with
time, with the possibility of activity continuing across the
southernmost Rockies and terrain of southeast AZ until the end of
the period as the dynamics strengthen -- the 21z SREF suggests
that CAPE may not exhaust with 500+ J/kg as late at 12z Sunday.
An early season Atmospheric River is progged to lift onshore
beneath a 110+kt jet streak rotating into British
Columbia/Washington State late Saturday night. GFS progs for the
AR suggest a high probability for IVT 500-750 kg/m/s moving into
Washington, with PWATs exceeding 1.25" on 30 kts of 850mb flow
with 200+ J/kg of CAPE present. This robust moist advection will
be wrung out via ascent within the RRQ of the pacific jet streak,
combined with height falls and surface convergence along/ahead of
a mid-level trough driving a cold front onshore. This will
support rain rates which at times may exceed 0.5"/hr. As flow
briefly becomes parallel to the front, especially into the Olympic
Range of Washington, training and upslope enhancement could
produce isolated instances of flash flooding across the terrain.
The marginal risk area remains despite the expected short temporal
duration of favorable dynamics for excessive rain rates.
The Day 2 outlook will be updated by 0830Z.
The Day 3 outlook will be updated by 0830Z.
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt