Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
243 AM EDT Sat May 18 2019
Valid 12Z Tue May 21 2019 - 12Z Sat May 25 2019
...Overall Weather Pattern...
The upper level pattern across the continental U.S. will continue
to be rather amplified and slow to change next week. The upper
level ridge over the southeast U.S. becomes anchored over the
region and gradually becomes stronger for the second half of the
upcoming work week. Across the western U.S., a large scale trough
will be dominant as multiple shortwave perturbations keep
conditions cool and unsettled. This will also be slow to move
given the expected development of an omega block pattern over the
eastern Pacific. The main weather feature of interest will be a
strong low pressure system across the Plains and Upper Midwest for
the Tuesday and Wednesday time period, and then followed by a
weaker low and frontal boundary for the end of the week.
...Guidance/Predictability Assessment...
The models and ensemble means indicate the greatest agreement with
respect to the building upper level ridge over the southeastern
U.S., with the ECMWF and the EC mean supporting the strongest
solutions. The greatest differences involve the evolution and
track of individual closed lows within the western U.S. trough.
These become evident at the beginning of the forecast period on
Tuesday, with the 00Z UKMET notably more amplified with the upper
low over New Mexico, and its surface low tracks well to the west
of the model consensus. The CMC becomes more progressive with the
next closed low over the Desert Southwest and this becomes more
apparent by Thursday. Even greater differences exist with the
surface low that eventually reaches the Upper Midwest and southern
Canada by Thursday morning, with the ECMWF and EC mean over
Ontario and the GFS much more delayed in bringing the low out of
the Upper Midwest. The track of the ECMWF best fits continuity
and recent storm tracks. Towards the end of the forecast period,
there is an increasing model signal for a third trough axis to
close off near the California coast by Friday and Saturday, with
support from the GFS, FV3, and ECMWF. Taking these factors into
account, the WPC forecast was based on a blend of the
ECMWF/GFS/FV3/ensemble means, along with some of the previous WPC
forecast for continuity.
...Weather Highlights and Hazards...
Numerous showers and thunderstorms continue to be in the forecast
through the middle of next week from the central plains to the
Upper Midwest. Multiple rounds of MCS activity are likely with
both of the low pressure systems that are progged to track across
the Plains. This will have the potential to produce very heavy
rainfall capable of producing flash flooding, and severe weather
is also likely across much of the Midwest states based on recent
SPC forecasts. Additional heavy rain is also possible from the
next storm system tracking eastward from the Intermountain West
later in the week, although details on timing and placement
continue to remain unclear at this time.
There will also be a dichotomy of temperature anomalies given the
amplified flow pattern in place. Temperatures are expected to be
well above normal across much of the southeastern U.S. owing to
the strength of the upper ridge, and this will also serve to keep
rainfall chances minimal. Afternoon high temperatures may
approach 100 degrees for parts of Georgia and South Carolina by
later in the week. The opposite will be the case across much of
the western U.S. and also the northern Plains with readings on the
order of 10 to 20 degrees below climatology, which will be under
the influence of the upper level trough and to the north of the
low tracks. Heavier late season snows are also likely for the
Sierra Nevada to the northern Rockies for the middle of the week.
D. Hamrick
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