Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
237 AM EDT Thu May 23 2019
Valid 12Z Sun May 26 2019 - 12Z Thu May 30 2019
...Record/Dangerous Heat for the Southeast will continue into next
week...
...Heavy rainfall possible for portions of the central Plains and
mid-Mississippi Valley...
...Overview and Guidance Evaluation/Preferences...
Upper pattern will only slowly trend toward a bit less
amplification but otherwise maintain troughing in the west and
ridging in the Southeast and an active northern stream through
Canada. The models/ensembles have struggled to fully resolve the
flow out of the Pacific/high latitudes this weekend which keeps
shifting either (or both) the track or evolution of systems in the
eastern half of the CONUS. Ensemble means have wavered on either
slower/more coherent upper shortwaves or closed lows vs
quicker/strung out impulses which may or may not loosely phase
with northern stream flow over southern Canada. Best approach has
been that of avoiding extremes (too quick/slow or weak/deep) given
the lack of continuity over recent days. The 18Z GFS and 12Z
ECMWF/Canadian offered better clustering near the ensemble means
and formed much of the starting point. By next Wed/Thu only the
ECMWF was even somewhat close to the favored pattern though the
GFS/Canadian could not necessarily be discounted in totality. This
resulted in a bit quicker progression of the system out of the
west toward the Great Lakes Mon/Tue as well as with a northern
clipper system Tue/Wed. One constant has been keeping some frontal
boundary over the southern Plains through the period, reinforced
by upstream height falls and kept from pushing east due to the
strong upper high in the northeast Gulf.
...Weather Highlights and Hazards...
The heat is on. Widespread temperatures in the upper 90s and a
modest area of 100-103F will challenge monthly records in the
Southeast (AL/GA/n FL/SC). Heat indices over 100F are likely for
many areas under the upper high with no chance of rain due to the
stable atmosphere. Variable heat may extend into the mid-Atlantic,
perhaps with a push into the low/mid 90s up to DC next Wednesday.
Still a large spread in ensemble forecasts (60s to 100F) but as
the position of the front and intensity of lower level warmth is
still uncertain. Farther north, Temperatures will trend toward
average along the US/Canadian border where rain chances will be
greater as well.
Well-below average temperatures in the west will slowly ease by
the end of the period (next Thu) but abundant cloud cover and
modest rain (with some high elevation snow) will support some
record low max temperatures. To the east, temperatures around the
wavy boundary from the central Plains eastward will average closer
to normal and provide a focus for rainfall and some convection.
Rounds of rainfall with locally heavy amounts may add up to
several inches over the period, some of which may fall within a
short amount of time over areas that have seen quite a bit of
rainfall recently. Greatest flooding/flash flooding risk during
the period seems to be centered from eastern Kansas/Oklahoma
northeastward through Iowa on different days.
Fracasso
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