Area Forecast Discussion
Issued by NWS Quad Cities, IA IL
FXUS63 KDVN 241122 AAA
Area Forecast Discussion...UPDATED
National Weather Service Quad Cities IA IL
622 AM CDT Fri Mar 24 2017
...12z AVIATION UPDATE...
Issued at 246 AM CDT Fri Mar 24 2017
Early this morning a 992 hPa surface low was centered over Lamar,
Colorado. A surface trof extended ne from this low into western
Iowa. Aloft, a strong H85 LLJ with speeds from 50 to 60 kts was
fostering strong WAA into the area. Temperatures have jumped
nearly 10 degrees in the past 4 hours in Davenport. This low, and
the warming temperatures are the main forecast concerns in the
short term period.
.SHORT TERM...(Today and Tonight)
ISSUED AT 246 AM CDT Fri Mar 24 2017
Closed H5 low will slowly lumber to the east through the period as
a H25 jet streak translates north across the area. Model consensus
on precip chances is high, leading to a high confidence forecast
for precip. Precip should overspread the area from 21z to 03z
Saturday. As the jet streak moves north, intensification of the
precip should occur between 00z and 06z from south to north across
the area. This is reflected in some of the CAM runs as well. Main
questions with this system are how much QPF, chances for thunder
and the strength of convection.
1. How much QPF; through 00z Saturday consensus in the guidance is
for around a 0.10 of an inch mainly west of the Mississippi River.
Confidence is high in this occurring. After 00z, convective
elements associated with the jet streak across the area leads to a
higher spread in QPF. This means confidence in QPF after 00z
Saturday is lower and could range significantly, especially in
areas where convective elements make it through.
2. Chances for thunder; While overall model CAPE values are very
low, the dynamic nature of the system along with strong H5
divergence should lead to schc to chc thunder across the area,
especially this afternoon and evening as daytime heating is
maximized. Think the best chances for thunder will be associated
with the jet streak moving through the area later today.
3. Strength of thunderstorms; no severe weather is anticipated
with the low CAPE and low shear. H85 jet drops in magnitude
dramatically during the day today. As a result any organized
strong storm will need to be close sfc boundaries, non of which
are forecast across the area.
.LONG TERM...(Saturday through Thursday)
ISSUED AT 246 AM CDT Fri Mar 24 2017
Moisture transport will continue through Saturday, with widespread
rains expected, especially along and north of the warm front, which
is should be generally be located near Interstate 80. The upper low
moving toward the region will bring strong ascent through the day,
and as the dry slot nears the southern CWA by afternoon, there may
be a threat for shallow supercell thunderstorms along and just
south of the surface front. The threat for this is contingent on
surface heating near the boundary, and that is not yet a confident
feature in what could be a stratiform rain plagued environment.
None the less, if surface heating can attain mid to upper 60s with
dewpoints that should be in the mid to upper 50s, we could see
some mini supercell storms in the southern counties. Those storms
would likely carry a tornado threat along the boundary, and this
will need close attention Saturday afternoon and early evening.
Along and north of the boundary, with rains expected all day, while
not heavy the duration, it should be cool and damp, and a day held
to the 40s entirely possible. For now, with the boundary placement
not yet certain, I have gone near guidance south for highs, and
below guidance north. The counties near I-80 will have a large bust
potential as they could reach the upper 50s, or remain in the upper
40s, pending how far south the boundary lays out. It will be lake
enhanced as noted in previous AFD issuance.
From Saturday evening through Sunday, the system will be occluding
overhead. That should result in less thunder, and a continuation of
light rain/drizzle/fog/just plain old damp weather. I have continued
pops on the higher side of guidance through Sunday, as there should
be plenty of opportunity for measurable light rain/drizzle through
Sunday afternoon. In fact, if, (big if), any sun were for be allowed
Sunday, we could have additional shallow convective showers form.
Lacking the shear of Saturday, they should not have severe weather
threats if they form.
Monday another synoptic low should move past the region, and with
another closed mid level low pressure, I expected there to be a
solid rain shield moving along with the 850 to 500 low pressure
circulations, well north of the southern state`s convection.
The weather remains active, and our guidance blend brings pops in
again by Wednesday afternoon again, after a dry Tuesday and Tuesday
night, lingering through Friday. There is little doubt that the wet
weather will bring rises to rivers, but we are just now beginning to
look at the impacts of this. I will leave this to the next shift to
comment on further as the RFC works through some of the longer range
QPF to suggest river impacts.
.AVIATION...(For the 12Z TAFS through 12Z Saturday Morning)
ISSUED AT 622 AM CDT Fri Mar 24 2017
VFR conditions are forecast through the next 10 to 12 hours at the
TAF sites. Rain and potential thunderstorms will move into the
area near 00z Saturday. At this time, cigs and vsbys will drop to
MVFR and possibly IFR overnight. With low confidence of IFR
occurring and it being after the next 6 hours, decided to leave it
out of the forecast.