155
FXUS65 KBOU 250949
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
349 AM MDT Thu Apr 25 2024

.KEY MESSAGES...

- Slight Risk of severe thunderstorms this afternoon and evening,
  mainly east of a line from Sterling to Akron, including large
  hail and isolated tornadoes

- Showers and storms become more numerous late this afternoon and
  across the northeast plains through the overnight

- Fire weather concerns east Central Colorado

- Significant snowfall likely (70-80% chance) in the mountains
  Friday night through Saturday night, widespread rain showers
  over the plains

- Warmer and drier early next week

&&

.SHORT TERM /Through tonight/...
Issued at 349 AM MDT Thu Apr 25 2024

Main focus is on convective and severe storm threat.

But first, current satellite imagery shows low clouds wrapping
around a surface cyclone in southeast Weld County. We think there
is enough oomph in the northerly flow on the back side of this
cyclone per current satellite imagery and observations to push it
into portions of north Denver metro before daybreak. There could
even be some fog on the edge of this with narrow T/Td spreads,
unfortunately potentially impacting DIA operations. Otherwise, the
fog threat seems limited by a relatively high stratus deck thanks
to mild overnight low temperatures and earlier cloud development
over the eastern plains. The upslope component and cyclone is
quite shallow per the latest Vad Wind Profiler (VWP) data, so a
relatively quick burnoff is expected.

Now, moving onto the main focus today, the severe thunderstorm
potential. We saw little change in the SPC severe convective
outlook with a Slight Risk over the northeast plains including
Logan, Sedgwick, Phillips, and northern Washington Counties, and
we think that`s a good spot. That seems to be where the main
combination of CAPE and shear will come together. However, there`s
a considerable amount of uncertainty whether threat will be
realized. That`s due to a relatively narrow corridor of
significant MLCAPE (1000-1600 J/k) on the edge of the burning
stratus/cooler temperatures over the far northeast plains this
afternoon. So questions remain...Where will there be both enough
heat and moisture to break the cap? Also, will those storms be
able to root themselves into the higher instability airmass, or
simply move off to quickly to the northeast into a more stable
boundary layer for less of a severe threat?  The ingredients are
there for severe storms, including the MLCAPE, strong deep layer
shear, and even enough 0-1 and 0-3 km helicity (and updraft
helicity) for strongly rotating updrafts. The main threat appears
to be more into the large hail potential, but with the low level
shear and potential convective development along the surface
convergence/vorticity zone along/north of the dryline and warm
front interface we do think there is a threat of a tornado or two.
When is the threat? That`s another good question as a couple of
the convective allowing models suggest 3 pm for initiation time,
but others have more widespread and stronger storms late in the
evening when the stronger upper level support arrive and the cap
fully breaks. The latter is a plausible scenario as well. The main
point is severe storms are possible anytime from late afternoon
through the evening over the northeast plains. The main threat
area would be along and east of a Sterling to Akron line. However,
there is a threat farther west toward Fort Morgan or even a
little further into eastern Weld County should the later scenario
unfold...or...the dryline not fully retreat - and that would not
be surprising considering strong pressure falls into southeast
Colorado this afternoon.

Farther west including the mountains and I-25 Corridor, generally
weaker convection is expected. In fact, the odds of afternoon
showers and storms along the I-25 Corridor were reduced somewhat
as the airmass is quite dry so mostly high based convection
expected. There is an opportunity for stronger convection and
brief heavy rain after about 6 pm as the mid level front arrives.
That expected band of more widespread precipitation then shifts
eastward across the plains overnight, leaving areas west of I-25
mostly in downslope and less chance of precipitation.

Mountain areas will see more numerous rain/snow showers develop
this afternoon with the arrival of deeper upper support and
moisture. A mid level front late could bring a brief period of
heavy snow, but overall impacts outside of the mountain passes should
be limited by relatively warm road temperatures. Average amounts
of 2-5" expected above 10,000 feet through the night, with a
dusting to 2" down to about 8500 feet.

&&

.LONG TERM /Friday through Wednesday/...
Issued at 349 AM MDT Thu Apr 25 2024

On Friday, the first system will be over central NE by 18z with
weak mid/upper level subsidence over the northeast plains through
the morning. In the mountains, a short wave ridge will move
across the region through midday Friday. The best precipitation
coverage will likely be across the far northeast plains Friday
morning, with light snow showers in the mountains.

Friday night through Saturday, widespread precipitation will
develop across the forecast area. Increasing mid/upper level QG
ascent will develop late Friday, as the next short wave trough
approaches from the Great Basin. Moderate to strong QG ascent will
pass across central/southern CO on Saturday, with deep moisture
across the area. Moist adiabatic lapse rates, with potential
snowfall rates of 1-2"/hr in the mountains on Saturday. Snowfall
will be generated primarily by synoptic scale lift and
instability during the day Saturday with the snow level around
8000 feet. Saturday night into Sunday, the closed low will lift
into northeast CO by 06z Sunday and into central NE by 12z Sunday.
North to northeast mid level winds may start to favor the east
slopes of the Front Range Saturday night into Sunday morning. The
QG ascent will weaken overnight Saturday, with weak QG descent
developing for Sunday morning. Snowfall Saturday night into Sunday
will favor the northwest facing slopes especially Sunday
afternoon into Sunday night, but the coverage should start to
decrease. Mid level lapse rates on Sunday should still be decent,
with 700/500 lapse rates above 7C/km in the afternoon. 48-hr qpf
from 12z Fri to 12Z Sunday, still show totals to around 2 inches
expected. In the higher mountains, there is the potential of 1 to
2 feet of snowfall, heaviest from 06z Saturday to 12z Sunday.
Consequently, a Winter Storm Watch will probably need to be
hoisted at some point, but it`s still in Day3 ish time frame so
no highlight yet.

It will be drier and warmer by Tuesday, but a chance of showers
will continue in the mountains on Monday. Tuesday into Wednesday,
high temperatures will climb back to near 80 for the plains with
at least slight chance of thunderstorms Tuesday afternoon/evening.

&&

.AVIATION /06Z TAFS through 06Z Friday/...
Issued at 1159 PM MDT Wed Apr 24 2024

Main concern revolves around fog potential at KDEN early this
morning. Surface dewpoints and moisture were actually ahead of
schedule and higher than some model guidance. There is a Denver
convergence zone centered about 20 miles north of KDEN, with low
level moisture wrapping around that feature. We are seeing stratus
develop to the north and east, and this will effectively wrap
around the surface cyclone. Right now, it still appears KDEN will
be right smack on the edge of the fog from around 10Z-15Z, and
just a couple miles could make or break this forecast. There`s
about a 30-40% chance KDEN will sink to 1/4SM FG VV001 for a
couple hours centered around 12Z, but slightly more likely
(60-70%) that we stay with VCFG and low clouds on the north edge
of KDEN property. Stay tuned! If the fog does move in, then look
for erosion by ~15Z as there`s agreement of a more southerly flow
regime allowing a weakening or shearing of the cyclone to the
northeast.

Otherwise, VFR conditions will be the rule through tonight. With
regard to shower/storm potential, it looks rather limited 21Z-03Z
per the limited moisture once the cyclone breaks down and shears
east/northeast. Thus, we`ll go with VCSH for this afternoon and
evening. The best chance of rain will occur after 04Z when deeper
north-northwest flow develops, and that`s when IMC ceilings will
be more likely at KDEN. KBJC and KAPA could escape without much
shower activity being closer to the downslope off the Front Range.

Winds...West/northwest winds may become variable as the cyclone
shear out after 15Z. Then there should be a trend (considerable
uncertainty) for a more easterly wind component 19Z-23Z, until
the cold front and/or shower outflows arrive (most likely 00Z-01Z)
with gusty northwest to northerly winds up to 35 knots.

&&

.FIRE WEATHER...
Issued at 349 AM MDT Thu Apr 25 2024

Gusty southerly winds and a dryline is expected push back
northward across the Palmer Divide and Lincoln County area by
afternoon. Behind that, gusty south winds up to 30-35 mph and
humidities dropping to 10% or less can be expected. We are unsure
about fuels in that area, but taking a look at CoCoRaHS data there
has only been about 0.50 to 1.25" of precip there since March 1,
and only 1.5 to 2.5" since the first of February. The amount of
greenup is limited, so we`ve opted to issue a Red Flag Warning
considering the significant drop in humidity today.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 8 PM MDT this evening for
COZ246-247.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Cooper
AVIATION...Barjenbruch
FIRE WEATHER...Barjenbruch