Scientific Forecaster Discussion
Area forecast discussion
National Weather Service St Louis MO
352 am CDT Thu Apr 2 2015
Short term: (through tonight)
issued at 347 am CDT Thu Apr 2 2015
As has been discussed at length over the last several forecast
cycles, a couple of rounds of thunderstorms will likely affect the
County Warning Area today and tonight ahead of a strong cold front. The front then
stalls across the region tonight, raising a concern for heavy
rainfall and flooding.
The first round of thunderstorms will likely occur this morning
when the remnants of an upstream thunderstorm complex move into
the County Warning Area. Expect a weakening trend with time this morning due to
the decreasing low level jet. Morning precip is more likely in central MO,
northeast MO, and west central Illinois compared to the rest of the County Warning Area.
The next round is expected when thunderstorms (re-)develop this
afternoon. Instability will be increasing today due to increasing
low-level moisture and steepening 700 mb-500 mb lapse rates. Where there
are breaks in the cloud cover today, the resulting diurnal heating
will further increase the instability. Model forecasts of 0-6km
shear are sufficient for organized convection and for strong to
severe thunderstorms, especially between 21z-03z and particularly
near and south of the I-70 corridor. The trend will be for
convection to quickly become linear since 0-6km shear vectors are
oriented and remain nearly parallel to the approaching cold front.
The last round of thunderstorms appears to be less of a distinct
round and more like an expansion of the afternoon convection into
a broader area of rain and thunderstorms which then persists
overnight (especially across the southern cwa) before tapering off
to showers on Friday.
Model forecasts of precipitable water values near 1.3" are at +2sd for April in
this part of the country, which by itself increases the concern
for heavy rainfall. Furthermore, the cold front becomes oriented
east-west and then stalls overnight while a new surface low
develops on the tail end of the boundary in Kansas/OK/TX in response
to an approaching shortwave (located near the California/Nevada border at
08z). With mid/upper flow nearly parallel to the boundary and a
developing surface low on the tail end, it will be difficult for
the front to make much southward progress until the shortwave
passes. Training thunderstorms are possible overnight invof this
stalled boundary. A Flood Watch has been issued for the southern
County Warning Area based on these factors.
Long term: (friday through wednesday)
issued at 347 am CDT Thu Apr 2 2015
The California/Nevada shortwave moves across the area on Friday, leaving the
region in northwest flow aloft for the first half of the weekend. Its
passage will also push the stalled front south of the area,
bringing much cooler air back into MO/IL. Expect highs on Friday
to be about 20 degrees cooler compared to the previous couple of
days. Some frost is possible on Friday night/early Saturday
morning with lows dropping into the low to mid 30s and decreasing
winds due to an approaching surface high pressure center. Cooler
weather persists on Saturday followed by moderating temperatures
on Sunday and into early next week with the return of southwest
flow around the back side of the aforementioned sfc high.
Models depict a weak and disorganized vort Max which lifts from the
Pacific through Mexico and into Oklahoma on Sunday, which could
bring a chance of rain to the area as early as Sunday night. A much
stronger low pressure system is forecast to move through the central
Continental U.S. Early next week and could bring thunderstorms to the area on
Aviation: (for the 06z tafs through 06z Thursday night)
issued at 1104 PM CDT Wed Apr 1 2015
Showers and a few thunderstorms ahead of an upper level disturbance
just south of uin should shift southeast of uin by 06z Thursday.
A strong swly low level jet will lead to low level wind shear late tonight with
S-swly winds of 40-45 kts around 1500-2000 feet in height. A line
of convection across the northern plains will move through the taf
sites Thursday morning. This convection is ahead of a cold front
which will be dropping southeastward into northwestern MO Thursday
morning. This activity should be gradually weakening during the
morning hours. The prevailing cloud ceiling will likely drop into
the MVFR catagory Thursday morning as the low levels saturate.
Another round of convection, possibly severe is expected mainly
during the late afternoon and early evening in cou and the St
Louis Metro area along the cold front as it continues to move
slowly southeastward. The surface wind will veer around to a
N-nwly direction Thursday afternoon or evening after fropa.
Specifics for kstl: strong winds of 45 kts at 1500-2000 feet in
height will lead to low level wind shear conditions late tonight. Showers and
storms can be expected Thursday morning as a weakening line of
convection drops southeastward into the area ahead of an
approaching cold front. Another round of stronger convection is
forecast late Thursday afternoon into the evening along the cold
front. This convection should weaken later Thursday evening with
lingering showers, while the surface wind veers around to a nly
direction after fropa.
MO...Flood Watch from this evening through Friday afternoon for
Crawford MO-Iron MO-Madison MO-Reynolds MO-St. Francois MO-
Ste. Genevieve MO-Washington MO.
Illinois...Flood Watch from this evening through Friday afternoon for
Randolph Illinois-Washington Illinois.
Weather forecast office lsx