Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
338 PM EDT Tue Apr 16 2019
Valid 12Z Fri Apr 19 2019 - 12Z Tue Apr 23 2019
...Heavy rain threat for parts of the east Friday...
...16Z update...
The narrow trough moving across Mississippi Valley on day 3
appeared to be better handled by the 00Z and 06Z GFS than in
previous runs. Deterministic models and means across keep one
trough in both the northern and southern stream at day 3, with the
vort max then spinning into an upper low in the southern stream by
day 4. Thus the medium range forecast used a multi-model blend
through day 3-4. This pattern still supports heavy rain across the
East early in the period.
Split flow remains an issue with the latest guidance across the
western/central U.S., and once again leaned away from the
operational GFS during the later forecast period as it strengthens
and slows down a trough in the western U.S., unlike consensus and
suggestion from teleconnections. Favored the EC/NAEFS/GEFS means
with some element of the operational 00Z ECMWF.
For details on sensible weather, please see the previous
discussion.
Tate
...Previous Discussion...
...Overview...
Quasi-progressive pattern for the medium range as atmospheric
spindown continues during this transition season. Deep
trough/closed upper low will push east of the Mississippi late
this week and perhaps cut off from the flow off the
Mid-Atlantic/New England coast as western troughing (from both the
subtropics and the mid-latitudes) push out of the Rockies and onto
the Plains next Mon/Tue. Heavy rain threat at the end of the short
range will shift to the Appalachians --> coastal plain on Friday
into early Saturday.
...Guidance Preferences...
Models and ensembles continued to struggle with exactly how the
eastern system evolves during this period, contingent upon the
degree of northern/southern stream separation and exact track of
the upper features. 12Z/18Z GFS runs seemed to depart most from
the otherwise modest clustering of the ensembles and 12Z
ECMWF/UKMET/Canadian with the track of the upper low (GFS was
farther north and thus quicker to bring the system back into the
westerlies than the ECMWF-led cluster). Preferred to side with the
non-GFS guidance given the trend in especially the ECMWF ensembles
but also the GEFS members. Nature of the flow does not instill
high confidence in the details given the sensitivity to shifts in
the short range forecast, so the GFS was still within the realm of
possibilities.
To the west, ensemble consensus has proven its worth in minimizing
shifts in the forecast over the past several days. Again here, the
GFS departed from the majority consensus with the trough in the
Southwest late Sunday (slower vs a quicker consensus). With split
flow out of the Northeastern Pacific, uncertainty increases in the
forecast, necessitating an increase in the ensembles.
...Weather Highlights/Threats...
Deterministic models continue to paint stripes of modest/heavy
rainfall northeastward along and east of the Appalachians as the
flow becomes increasingly meridional. This should focus a
relatively narrow but intense plume of moisture from the
subtropics (Yucatan/Cuba/Greater Antilles) along/ahead of a cold
front. With PW anomalies +3 to +4, rainfall could be lead to
flooding concerns, especially in urban areas. Severe weather
threat will accompany the front, likely focused south of the
Mason-Dixon line (please consult the SPC outlooks for more
information). GEFS mean QPF continues to be much higher than
typical values for this lead time in mid-April, suggesting a
higher likelihood of an anomalous event.
Mild temperatures will precede the front (10-15F above average)
but will be replaced by cooler than average temperatures after its
passage. That will be short-lived (1-2 days) as milder air in the
west Friday pushes through the Plains this weekend and into the
east early next week.
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
Hazards:
- Heavy rain across portions of the Southern Plains, Tue, Apr 23.
- Heavy rain across portions of the Northeast, the
Central/Southern Appalachians, the Mid-Atlantic, the Southeast,
the Great Lakes, and the Ohio Valley, Fri-Sat, Apr 19-Apr 20.
- Severe weather across portions of the Southeast, the
Central/Southern Appalachians, and the Mid-Atlantic, Fri, Apr 19.
- Flooding possible across portions of the Northeast, the Northern
Plains, the Tennessee Valley, the Southern Plains, the Northern
Great Basin, the Upper Mississippi Valley, and the Great Lakes.
- Flooding occurring or imminent across portions of the Northeast,
the Northern Plains, the Mississippi Valley, and the Ohio Valley.
- Flooding likely across portions of the Southeast, the
Mid-Atlantic, the Northeast, the Upper Mississippi Valley, and the
Northern Plains.
- High winds across portions of the Central/Southern Plains,
Sun-Mon, Apr 21-Apr 22.
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