Scientific Forecaster Discussion
fxus63 kdmx 201154
Area forecast discussion
National Weather Service Des Moines Iowa
544 am CST Wed Nov 20 2019
Short term.../today through Thursday/
issued at 411 am CST Wed Nov 20 2019
Patchy fog developed across the northeast overnight. Most
visibility observations ranged between one and four miles but
locally, visbilities have dropped briefly to a quarter mile.
Through mid morning the fog should be confined to eastern
Iowa...possibly affecting Waterloo but the lower visibilities will
remain over eastern Iowa.
For the rest of today, an upper ridge will shift east with the ridge
axis east of the forecast area by late in the day. A shortwave
ejecting out of a deep trough over the southwest U.S. Will ride up
into the plains by late afternoon impacting southwest Iowa after
21z. At the surface, low pressure will develop and lift a warm
front across west central to southeast Iowa. Notably warmer
temperatures will occur south of the boundary today with highs in
the lower 60s, despite increasing cloud cover. Forcing increases
late afternoon and especially tonight in conjunction with a stronger
shortwave pushing out of the Central Plains and into Iowa. The
aforementioned surface low will push further into Iowa during this
time lifting the warm front into northeast Iowa. A narrow tongue
of instability will work into mainly southwest Iowa then traverse
across southern Iowa during the evening. A rumble or two of
thunder cannot be ruled out over these areas.
During the overnight hours tonight, the surface low lifts to the
northeast and drags a cold front across the state. Precip will
quickly end from southwest to east across the state with only
lingering precip across the far north and east Thursday morning.
Rain may mix with snow briefly before ending Thursday morning. Not
expecting any accumulation...certainly nothing impactful.
By Thursday afternoon surface high pressure builds into the area and
drier air moves in ends precip with some clearing expected west.
There will be quite a contrast in temps with much colder air across
the northwest. Highs will vary from the mid 30s northwest to the
upper 50s far southeast.
Long term.../Thursday night through Tuesday/
issued at 411 am CST Wed Nov 20 2019
/Thursday night through Sunday night/...
A well advertised pattern change will be ongoing for much of the
seven days with the first system occurring in the short term
forecast. Although the models are in very good agreement in the
short term, forecast confidence has progressively decreased toward
the end of Sunday and for much of next week.
Initially behind the Wed night/Thursday system, a secondary northern
stream trough will pass south into the area by 12z Friday while
another h500 wave over Colorado ejects a shortwave over the Southern
Plains. This may bring enough cloud cover and isentropic
lift/mechanical lift into southern Iowa between 18z Friday to 06z
Saturday as the h500 wave lifts to northern Kansas that a slight
chance of light rain/snow is still warranted over the far south
during this time. Lows Thursday night will be chilly in the upper
teens north to the mid 20s south. Friday overall will be quiet
prior to the approaching southern stream wave with afternoon highs
in the mid to upper 30s. Any increase in cloud cover should be
confined mainly to the south. On Saturday the system will pull
east and quieter weather will return once again. With both the
Euro and GFS showing a weak northern stream wave again progressing
east north of our area; slightly milder temperatures are expected
for Saturday into Sunday with highs both days in the 40s to around
50 on Sunday. Unlike yesterday, the models now are featuring a
more progressive and continued fast west northwest flow into
Sunday night and early Monday with little western trough
amplification just yet.
/Monday through Tuesday/...
The Euro and GFS both drop a shortwave into the Central Plains by
Monday with the GFS rapidly deepening a surface low into Tuesday -
though it looks to be suffering from convective feedback. The euro;
however, now is keeping the system progressive with a much less
amplified system crossing into the Great Lakes. Over the course of
the next several days into Thanksgiving, both the GFS/Euro suggest a
more widespread h500 trough anchoring over the central Continental U.S.. during
the week, numerous pieces of energy are progressing through the
flow. It appears that the models are having a lot of difficulty
resolving the systems and subsequent amplification. The leading
wave that approaches Monday in each model will probably determine
the outcome for the remainder of next week and depending on how much
cold air precedes the main jet Max approaching into Tuesday and
beyond. Given that we are now entering the 120 hour forecast period,
it's not much of a surprise that the signal is now rather muddled.
With the large change in the pattern expected, it would not be
unusual for a more organized system to again show up over the next
few days in the Euro and a day or two of change yet in the GFS as
well. At least temperatures are probably a bit easier to forecast
based on h500/h850 heights. The colder conditions should yield
highs colder in the 30s to lower 40s Tuesday and colder yet for the
remainder of next week. With the more active pattern expected,
precipitation chances are also on the upswing from Monday through
Aviation.../for the 12z tafs through 12z Thursday morning/
issued at 544 am CST Wed Nov 20 2019
Cig/vsby restrictions due to fog will impact mainly kmcw and Kalo
through 14z with IFR/LIFR conditions. After 14z VFR conditions are
forecast. A storm system will move across taf locations after
21/00z. Conditions will deteriorate to IFR/LIFR conditions once
again. Low level wind shear is forecast mainly between 21/00z and 21/06z.