Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
154 AM EDT Sun Jun 30 2019
Valid 12Z Wed Jul 03 2019 - 12Z Sun Jul 07 2019
...Overview and Guidance/Predictability Assessment...
Typical pattern for summer expected across the CONUS during the
medium range. Downstream of a Rex block across the north Pacific,
moderately progressive flow will pass between a building upper
ridge across the CONUS southern tier, and a broad upper low in the
vicinity of Hudson Bay. A wavy surface front will persist from the
northern/central Plains to the Great lakes through much of the
forecast period as a number of relatively small-scale shortwaves
traverse the upper jet from the northern Rockies to the Great
Lakes/Northeast. The front should be reinforced by the arrival of
another polar front by midweek.
Due to the establishment of the persistent Rex block and the
resultant increase in stability of the large scale pattern, model
solutions showed above average consensus at the synoptic scale
through much of the medium range forecast period. A multi-model
deterministic blend including the ECMWF/GFS/UKMET/CMC was used as
a forecast starting point for days 3-6 (Wed-Sat). These solutions
showed relatively modest timing and amplitude differences with
shortwave energy moving from the Pacific Northwest eastward across
the northern Rockies to the northern Plains as it the energy
begins to shear and interact with additional energy rotating
around the Hudson Bay upper low. By day 7 (Sun) the primary
question remains how the upper flow will evolve across the
northwestern U.S. downstream of the Rex Block. The GFS has
consistently shown an upper trough persisting while the ECMWF
develops a ridge axis. Teleconnections associated with the
anomalous Alaska upper ridge support either flat flow or some
degree of troughing across the Northwest, and this scenario is
generally supported by the ensemble means. Thus trended to
majority ECENS/GEFS ensemble means for day 7 to account for the
increased uncertainty.
...Weather Highlights/Threats...
The wavy surface front extending from the northern Rockies to the
Great Lakes, and a second cold front arriving by the middle to
latter portion of the week will focus areas of showers and
thunderstorms across multiple days, with areas of heavy rainfall
possible, especially from the northern/central Plains to the Upper
Midwest. Scattered showers and thunderstorms will be possible
farther east Wed-Thu, from the Southeast to the Ohio Valley and
Mid-Atlantic, as a moist and unstable air mass remains in place.
Storms may become more widespread across these areas by Fri-Sat as
a shortwave crosses the Great Lakes pushing the surface cold front
eastward. High temperatures are expected to be 5 to 10 deg F above
average along the Eastern Seaboard through much of the period,
with heat index values over 100 deg possible across much of the
Southeast and southern Mid-Atlantic. High temperatures are
expected to be 5 to 15 deg below average from the northern Rockies
to the northern/central High Plains Wed-Sat due to widespread
precipitation and the cooler air mass north of the polar front
which should be moving southward across the region.
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