Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EDT Fri May 24 2019
Valid 12Z Mon May 27 2019 - 12Z Fri May 31 2019
...Record/Dangerous Heat expected across the Southeast...
...Heavy rainfall possible for portions of the Plains and
Mid-Upper Mississippi Valley...
...Overview and Guidance Evaluation/Preferences...
An amplified flow regime is expected to persist during the medium
range from the North Pacific across North America. The most
persistent large scale features will be a strong subtropical ridge
anchored near the Southeast U.S., a mean upper trough across the
West (likely becoming cutoff over time), and a persistent upper
low near Hudson directing an active northern stream upper jet into
the north central states.
While models and ensembles showed generally good agreement at
large scales even through day 7, fairly significant differences
emerged even relatively early in the forecast period with respect
to the details of individual systems. Energetic shortwave energy
moving from the Rockies into the central U.S. days 3-4 (Mon-Tue)
should spark development of a surface low pressure system across
the Central/Northern Plains and Midwest, with the potential for at
least a couple waves of low pressure along the front. As a result,
models showed fairly significant timing/placement differences for
these areas of low pressure across the north central U.S., which
grew further by day 5 (Wed) as the main surface low crosses the
Great Lakes and lifts north into Canada. A blend of deterministic
solutions was used as a starting point during this time frame,
using solutions most closely clustered with the ensemble means. A
blend of the 12Z ECMWF/UKMET/18Z GFS were used during days 3-4,
with the UKMET dropped on day 5 in favor of increasing weight on
ensemble means.
By days 6-7 (Thu-Fri), the main disagreement among the guidance
was exactly how much heights fall across the Midwest/Great
Lakes/Northeast as an energetic shortwaves moves around the larger
upper low near Hudson Bay. The 12Z ECMWF was much more amplified
with this feature than the GFS (and relative to previous ECMWF
runs). The ECENS and GEFS ensemble means, on the other hand,
showed very similar setups by day 7, with the GFS seemingly the
closes deterministic solution to the means. This idea keeps lower
heights confined farther north in Canada with a period somewhat
zonal flow across much of the central/eastern U.S. as the upper
ridge weakens (at least somewhat). Given the greater uncertainty,
the ECENS/GEFS means were weighted more heavily by days 6-7, along
with some continued use of the GFS.
...Weather Highlights and Hazards...
High temperatures are expected to be 10-20 deg F above average
across much of the Southeast Mon-Thu, with temperatures expected
to near or surpass 100 deg F for many areas along/below the fall
line. These temperatures will combine with moderate humidity
levels to produce dangerous heat index values in excess of 105 deg
for some areas. Meanwhile, the upper trough across the Great
Basin/Rockies early next week will be accompanied by cool
conditions and high temperatures 10-20 deg below average.
Elsewhere, heavy rain will pose a threat across portions of the
central U.S. through much of next week. The low pressure system
across the central U.S. could produce multiple rounds of
potentially heavy rainfall and severe thunderstorms Mon-Tue across
portions of the Plains and the Midwest, with multi-inch rainfall
totals possible. By Wed-Thu the Northern Plains and much of the
Midwest should dry out as the surface front moves east, but the
trailing end of the front is expected to linger across the
Southern Plains and lower/mid-Mississippi Valley where it could
continue to focus areas of showers and storms with at least
locally heavy rainfall.
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