Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1203 PM 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
from the Great Basin/Rockies to the central U.S. and Great Lakes,
where interaction with amplifying northern stream energy is a
possibility.
There is relatively good agreement amongst the various model
solutions for days 3-4. Beyond that, differences begin to arise in
some of the deterministic runs. The 06z run of the GFS is less
amplified/weaker with the Hudson Bay upper low which allows for a
quicker progression of the ridge into central Canada on days 6-7.
Farther south, it's also a bit faster with a shortwave exiting the
Mid-Atlantic on Day 5 compared to its previous runs, the ECMWF,
and the ensemble means. The Canadian is also on the quick side
with that wave, as well as much more amplified with shortwave
energy digging into the Great Lakes Sunday into Monday. Thus, the
06z GFS and the 00z Canadian were rejected. The ensemble means
show good agreement with each other as well as with the 00z ECMWF
throughout the entire medium range period.
This cycle of the WPC medium range progs were comprised using a
blend of 00z GFS, 00z ECMWF, and the EC and NAEFS ensemble means,
with more weighting placed on the 00z ECMWF than the 00z GFS
because of better run to run continuity. Increased the weighting
of the ensemble means by days 6-7 to mitigate some differences
with smaller scale details. This blend results in a forecast
reasonably close to that of the previous shift.
...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 be 10-20 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.
Santorelli/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