National Weather Service Forecast Discussion

Thornton Weather Forecast   
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000
FXUS65 KBOU 250153
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
753 PM MDT Wed Apr 24 2024

.KEY MESSAGES...

- Slight Risk of severe thunderstorms Thursday, east of a line
  from Sterling to Akron, including large hail, damaging winds and
  isolated tornadoes. A marginal risk south and west of this
  area.

- Cool and unsettled Friday into the weekend, with accumulating
  moderate snowfall in the mountains, widespread rain showers over
  the plains

&&

.UPDATE...
Issued at 742 PM MDT Wed Apr 24 2024

One thunderstorm is still doing its best to persevere in a
generally hostile environment along the CO/NE border this evening,
with just a couple of weak, high-based showers noted elsewhere.
Not expecting much overnight tonight with just some chance PoPs in
the far northeast corner overnight.

As better moisture streams into the area overnight, there should
be some development of stratus/mist in the eastern plains, along
with some patchy fog. Have also added a little bit of fog into
the grids across southern Weld county into the DIA area where
guidance the HRRR has hinted at this the past few cycles.

Otherwise, only some minor adjustments were made to the forecast
grids this evening. The previous forecast was largely in good
shape.

&&

.SHORT TERM /Through Thursday/...
Issued at 324 PM MDT Wed Apr 24 2024

Northern border shower activity is running a couple hours ahead of
schedule. The edge of the drier air is lifting northward and there
does appear to be more stable air upstream west of the Front
Range, so the end of this over the mountains may also be faster,
in the early evening or sooner. On the plains the showers will be
encountering the south winds that could keep them going longer,
although there`s not really more moisture out there yet. Further
south, there`s just a slight chance of very light showers, we`ll
probably just have virga.

A surface low will develop early Thursday on the broad shear zone
on the edge of the southerlies on the plains, probably starting
near Denver but then drifting eastward. Moisture will be
increasing in the southerlies tonight. There is a stratus deck
from western Kansas into Texas that will advect into the northeast
Corner of Colorado tonight, and it will likely develop westward
late tonight. The increase in moisture is gradual at first, but
richer air will arrive by Thursday morning. Between the wind
speeds and the existing stratus deck, it looks like this will
probably be off the ground instead of producing fog.

Thursday`s severe potential is complicated by this cloud deck and
the fact that the moist air is considerably cooler and should be
capped, at least for most of the day and possible into Thursday
evening. So the question becomes where there will be both enough
moisture and heat, or enough convergence to break the cap. If we
can get convection rooted in that moisture, it`s a pretty
favorable supercell environment with mainly a large hail threat. I
see two severe scenarios, both of which are questionable, and both
of which could happen together.

One is on the west edge of the better moisture, extending on the
north side of the surface low, probably across northeast Weld, or
possibly Logan or Morgan counties. This area could be the sweet
spot which has increased moisture but still gets warm enough.
While CAPEs in this area won`t be as high as further east, it`s
still a supercell environment and this area might have the best
potential for tornadoes early in the life of the storm with a more
backed low level wind field. This would most likely be for a
couple hours during the late afternoon. The second scenario would
be that the dryline convergence is enough to overcome a weak cap.
Most of the convective allowing models we`ve seen like this idea,
and insist on it happening in the late afternoon. This seems like
it might be too fast given the model soundings we`ve seen which
are mostly still pretty cool in the moist air in the late
afternoon. It may take increasing convergence along with peak
heating/moistening into the evening if that`s going to happen.
This would be more likely on the east side of the low, east or
northeast of Akron, and a few hours later. In this case also, the
severe threat is probably just a couple of hours and then we`re
dealing with elevated convection over the cooler moist air.

Behind the low during the afternoon, we`ll have a relatively dry
environment, though still probably enough moisture/instability for
some higher based weaker storms. The bulk of that may wait for
evening as well with some cooling aloft and moisture advection
from the north/northeast behind the low.

For now, minimal changes have been made as this was the general
forecast scenario we`ve had going. The main question remains the
heating of the moist air late Thursday.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 324 PM MDT Wed Apr 24 2024

As the backside of the cyclone brings northwest flow to the
northeastern plains, the threat of severe storms could continue
through midnight. Counties such as Logan, Sedgwick, and Phillips
have the greatest chance of additional severe storms while
elsewhere instability decreases overnight. Although a nocturnal
threat of severe weather isn`t new, those areas listed above
should remain weather aware Thursday evening until midnight. Light
snow prevails throughout elevations above 9k ft overnight into
Friday morning. With increasing snow and mid-level cloud cover
across the region Friday, this forecast package includes an
adjustment to afternoon highs as well Friday night lows. The next
trough axis approaches Utah quickly but Friday afternoon may
become a break in additional precipitation before the next
shortwave trough enters western Colorado. Therefore, kept a chance
of scattered PoPs (40-50%) along the urban corridor Friday
afternoon. WPC Super Ensemble QPF fields display at least
0.50-0.75 inches over majority of the CWA Friday night into
Saturday morning. It is likely areas across the mountains could
receive periods of light to moderate snowfall. If traveling to
mountain passes, prepare for winter travel conditions. Snowfall
rates could approach 1-2 inch/hr above 10k ft especially by
Saturday afternoon. Heavy rainfall will likely occur across the
lower elevations Saturday as precipitable water values 0.7-1
inches. Numerous showers continue through Sunday morning. It is
possible these showers will remain less convective as daytime
heating will be limited.

Our region will transition into zonal flow by Monday and weak
lingering mid-level moisture could lead to isolated showers
throughout Jackson and Grand counties. By Tuesday, dry conditions
continue across the region.

&&

.AVIATION /00Z TAFS through 00Z Thursday/...
Issued at 524 PM MDT Wed Apr 24 2024

VFR conditions should prevail through the TAF period. A broad
Denver cyclone has been observed again this afternoon with the
main boundary/wind shift located between BJC and DEN. Current
expectation is for this to remain fairly steady-state through the
next couple of hours, and then gradually weaken later this evening
as drainage flow establishes.

Next forecast challenge comes early Thursday as models hint at
some potential for a brief period of fog or very low stratus
tomorrow at DEN. BUFKIT profiles would favor fog, but the
majority of guidance keeps this just north/northeast of the field
in southern Weld county.

Finally, there`s still some uncertainty in the winds and overall
convective/shower potential Thursday afternoon, with DEN
reasonably close to the dryline. Will need to re-evaluate with
tonight/tomorrow AM hi-res guidance to get a better feel, but a
broad period of VCTS/VCSH is likely. Eventually may see some lower
cigs develop after 00z as well.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Hiris
SHORT TERM...Gimmestad
LONG TERM...AD
AVIATION...Hiris

NWS BOU Office Area Forecast Discussion