Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
232 AM EDT Tue May 21 2019
Valid 12Z Fri May 24 2019 - 12Z Tue May 28 2019
...Record/Dangerous Heat for the Southeast this weekend into next
week...
...Heavy rainfall possible for portions of the central Plains/Corn
Belt...
...Overview...
Highly amplified upper level pattern will be dominated by deep
troughing in the west and strong ridging in the Southeast. This is
forecast to largely hold in place as reinforcing height falls sink
into California on Saturday as the lead upper low lifts through
the High Plains. The Northeast/Mid-Atlantic will lie in the
confluence between the northern and southern stream as a wavy
boundary oscillates north/south. The pattern favors well-below
average temperatures in the west with rounds of precipitation but
hot/dry conditions in the Southeast with record temperatures near
100F. In between, broad southwesterly flow aloft and south to
southeasterly flow at the surface will favor rounds of convection
and locally heavy rainfall with multi-inch totals over the period
especially over the central Plains/southwestern Corn Belt.
...Guidance Evaluation and Preferences...
Latest guidance proved to be a good consensus/starting point for
the forecast, despite their continued differences right from
Fri/D3 with the system exiting through the High Plains. Opted to
rely entirely on the deterministic models to start given a good
clustering of solutions though the ECMWF ensembles were still
notably quicker. Incorporated more ensemble guidance by next
Mon/Tue as the track of the upper low in the west loses focus as
well as the strength of the upper high in the Southeast (ECMWF-led
guidance was generally stronger than the GFS-led cluster). 18Z GFS
and 12Z ECMWF offered some good details with the western system
next week but were both quicker than their ensemble means by next
Tue/D7 over IA/MN vs Nebraska. Ensemble trend has been much slower
so favored the means by the end of the period.
...Weather Highlights and Hazards...
Summertime heat will become increasingly intense in the Southeast
though at least humidity levels may be only moderate (dew points
in the 60s F) which would allow overnight lows to fall to the
upper 60s to low 70s but would support heat indices of 100-105F
during the day. Temperatures will approach daily and quite
possibly monthly records in the 90s to low 100s over portions of
northern Florida and Alabama/Georgia/South Carolina. ECMWF-led
guidance has been quite hot and several degrees above the record
hottest May temperatures for many locations ever observed, but
seemed too hot (105+) given the time of year and climatological
history. Regardless, precautions will need to be taken as
successive hot days repeat.
The west, by contrast, will see well-below average temperatures
(about 15-25F below typical values) with abundant cloud cover and
modest rain with high elevation snow. Temperatures around the wavy
boundary from the central Plains eastward will rise/fall as the
front lifts northward or slips southward. This will also 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 seems to be centered over eastern
Kansas northeastward through Iowa, but with a notable north-south
spread in the ensembles.
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