Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 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...
...1600 UTC Update...
The latest forecast is generally a refinement of continuity, with
a lead system ejecting from the northern High Plains and a deep
upper low dropping southward along the West Coast before ejecting
inland to produce another significant Plains system early-mid
week. An operational model consensus still looks good through day
5 Sun. Then models/ensembles diverge regarding how much flow cuts
into the eastern Pacific ridge aloft--in turn affecting
progression of the western U.S. trough/embedded upper low into the
Plains. At the moment the GFS/GEFS runs are much more aggressive
than most other guidance to bring height falls into the ridge and
thus become faster to eject the western trough. Given the pattern
amplitude, farther westward ensemble mean trend for the upper
trough/low into day 5 Sun (compared to 1-2 days ago), and typical
GFS/GEFS biases, preference leaned 70 percent toward the
ECMWF/ECMWF mean/CMC scenario by days 6-7 Mon-Tue. It is possible
for energy on the western side of the overall trough to help eject
the feature more quickly than the slowest solutions so partial
inclusion of the GFS/GEFS idea is warranted at least over the
CONUS though maybe less so over the eastern Pacific. Fortunately
into early day 7 Tue surface differences over the central U.S. are
not quite as dramatic as the spread aloft. At that time the
model/mean blend yielded a slightly slower trend for the central
Plains surface low. One other trend of note is for greater
amplitude of energy crossing the Northeast next Mon-Tue. This led
to a southward adjustment for the associated surface front over
the East.
Rausch
Previous Discussion issued at 0632 UTC...
...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