Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
140 AM EDT Mon May 27 2019
Valid 12Z Thu May 30 2019 - 12Z Mon Jun 03 2019
...Episodes of heavy rain possible for portions of the central
U.S...
...Overview and Guidance Evaluation/Preferences...
Split flow is expected to persist across North America during the
medium range. The subtropical ridge is expected to remain
entrenched across the Gulf of Mexico, keeping a feed of warm and
moist air into the central and southeastern states. Flow
associated with an active North Pacific upper-level jet will split
as it approaches the West Coast of North America where a ridge
axis will be in place through much of the period before gradually
shifting eastward to the central U.S./Canada by late in the
weekend into early next week. As northern stream energy
periodically encounters a persistent upper low in the vicinity of
Hudson Bay, the energy will amplify in the broad cyclonic flow
with shortwaves digging into the north central and northeastern
U.S. Meanwhile slower progressing southern stream energy will move
fro the Great Basin/Rockies to the central U.S. and Great Lakes,
where interaction with amplifying northern stream energy is a
possibility.
The 12Z ECMWF/CMC were the preferred deterministic solutions
through much of the medium range forecast period, and a blend of
these solutions was used heavily in the forecast for days 3-5
(Thu-Sat). The GFS and FV3 solutions differed from the start on
Thu with the system crossing the Midwest/Great Lakes. The GFS/FV3
were deeper with southern stream energy crossing the Midwest on
Thu, and slower with amplifying northern stream shortwave energy
south of Hudson Bay relative to the ECMWF/CMC. The result was a
more significant and slower wave of low pressure along the surface
front across the Midwest on Thu in the GFS relative to the
ECMWF/CMC. The UKMET sided with the ECMWF/CMC but had its own
issues and seemed too weak with the frontal wave and too fast with
pushing the cold front eastward. The ECENS/CMCE ensembles provided
a significant degree of support to their deterministic runs,
lending to the preference of those solutions over the GFS even on
days 3-4.
By days 6-7 (Sun-Mon), ensembles show good consensus that the
upper ridge axis initially across the Pacific Northwest and
western Canada should propagate eastward to the north central U.S.
and central Canada, allowing height falls to reach the
northwestern U.S. as a Pacific jet/shortwave arrive. The ECMWF and
CMC showed similar solutions to the ensemble means with respect to
his evolution, as well as with another shortwave rounding the base
of the Hudson Bay upper low into the Great Lakes, with an
associated frontal wave moving from the Midwest on Sat to the
Great Lakes/Northeast Sun-Mon. Detail differences and run-to-run
variability do increase among the deterministic guidance by late
in the forecast period. As a result, ECENS and NAEFS ensemble
means were boosted to comprise a majority of the forecast for days
6-7, with continued use of minority components of the ECMWF/CMC.
...Weather Highlights and Hazards...
Areas of showers and thunderstorms with locally heavy rain are
possible Thu-Fri from the Southern Plains to the Ohio Valley and
Northeast along a surface cold front and around the periphery of
the subtropical ridge. Another round of more significant and
widespread convection and heavy rainfall is possible across the
Southern/Central Plains Sat-Sun as the trailing end of the frontal
boundary lifts north as a warm front and weakens, and another cold
front dives south across the central U.S. before stalling by the
weekend and providing additional focus for areas of showers and
thunderstorms. Farther west, periodic showers are expected from
the Great Basin to the central/northern Rockies as a couple weak
southern stream shortwaves traverse the region.
Hot temperatures will persist into Thu across portions of the
Southeast and southern Mid-Atlantic, where highs may reach 10-18
deg F above average. A gradual cooling trend is expected from Fri
into early next week as the upper ridge retreats a bit into the
Gulf of Mexico allowing heights to fall across the Southeast -
although any cooling will be relatively modest with highs expected
to remain 5-10 deg above average.
Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml