Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1201 PM EDT Wed May 01 2019
Valid 12Z Sat May 04 2019 - 12Z Wed May 08 2019
...Overview...
Upper ridging will remain anchored over the northeastern Pacific
during the medium range period which will drive a split flow
across the Lower 48. A closed low in the southern stream will move
into California Sunday/Monday while northern stream systems skirt
along the US/Canadian border. Ridging off the Southeast coast will
wobble in place but likely build northward toward the end of the
period next Wed. Frontal boundaries in the east will waver as
surface waves move through.
...Guidance/Predictability Assessment ...
The models/ensembles have come into better agreement off the west
coast with the approaching upper low but still exhibit spread in
timing with the GFS generally quicker than the ECMWF and the UKMET
in between. Trend over the past 24 hours has been slower in the
GFS and again prefer a solution near the slower half of the
ensembles near the 00Z ECMWF ensemble mean but not as slow as the
deterministic ECMWF. By next Tue/Wed, trough axis is forecast to
move through the deserts with good ensemble agreement. Blend
between the 06Z GEFS mean and 00Z ECMWF ensemble mean by then was
a good starting point.
In the northern stream, a surface low will move through southern
Canada this weekend with a cold front across the High Plains and
Upper Midwest that will push through the eastern Great Lakes and
Northeast early next week. Differences in timing remain so a
blended solution was preferred again today. To its south, a wave
or two of low pressure will exit off the mid-Atlantic coast late
this weekend and drop its cold front into the Southeast. By next
Tue/Wed, that boundary will dissipate as upper ridging increases
and forces the thermal gradient to the front to the north. Tail
end of that boundary will linger across the central Plains into
the Corn Belt and is still unclear if a defined surface low will
exit through the Great Lakes or just remain a wavy front into next
week. For now, opted to maintain continuity with a wavy front
rather than depict a surface low like the ECMWF.
...Weather Highlights/Threats...
Front draped across the southern Appalachians back to the lower
Mississippi Valley will be a focus for locally heavy
rain/thunderstorms in the warm sector with another area of rain to
the north of the low across the northern mid-Atlantic and Southern
New England. This will keep temperatures below average on Sunday
along the PA/NY line eastward through CT/MA/RI in the 50s or low
60s. Cooler than average temperatures will remain in place over
the High Plains/Upper Midwest next week by 5-15 degrees F
(temperatures only in the 50s).
Closed low into CA/Southwest early next week will support
scattered showers and some convection from into the Great
Basin/Rockies as the height falls move eastward. Precipitation
(generally all rain with snow levels quite high) will mostly focus
on higher elevations but with some rain into the valleys,
dependent on mesoscale details. Moisture influx into the region
appears to be modest across southern CA/NV/AZ but drier mid-levels
may inhibit that potential rainfall, at least at the onset. Winds
will increase in tandem with height falls over especially higher
terrain as temperatures trend cooler than average. Precipitation
may expand over the Rockies around the middle of next week as the
upper low continues eastward. Southwest to westerly flow aloft
across the Plains next Tue-Wed will support showers and storms
along the northern stationary front to the north and near the Gulf
Coast as the southern stationary front moves northward.
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