650 
FXUS65 KBOU 200950
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
250 AM MST Sat Feb 20 2021

.SHORT TERM...(Today and tonight)
Issued at 228 AM MST Sat Feb 20 2021

Early this morning, a fairly strong northwesterly flow aloft is 
over CO with the ridge axis over the central Rockies. The next 
system is just off the OR coast, with a couple of lightning 
strikes already evident over northern CA. For today, the models 
shift the ridge axis over eastern CO/western KS by 18z, as the 
next system over the Pacific Northwest quickly makes its way over
southeastern ID/northern UT at that time. The RAP indicates 
increasing mid and upper level qg ascent developing over western 
CO, with a QG max of -36 mb/hr over central UT at 18z. This 
feature stays pretty well in tact around -25 to -30 mb/hr as 
quickly translates eastward across CO this afternoon and evening. 
This area will be an the left exit region of an 120+ kt jet which 
is progged to push into west central CO by 21z this afternoon, it 
then shifts southeast with strong mid level cold air advection 
developing from northwest to southeast across the forecast area 
this afternoon and evening. 

The combination of favorable instability, with 500-700 mb lapse 
rates of 8-8.5c/km, and increasing dynamics associated with a strong 
upper level jet could allow for a 3-6 hr window of moderate to heavy 
bands of snowfall. In the mountains, the strongest forcing and best 
instability appears to be from around 4 pm to 8 pm along and south 
of the I-70 corridor west of Denver. Storm totals in zones 33 and
34 are expected to range from 4-8 inches. Consequently, we have
issued a Winter Weather Advisory for zones 33 and 34, from 2 pm 
this afternoon until 5 am Sunday morning. Forecast soundings show 
surface capes 50-100 j/kg this afternoon, with moist adiabatic 
lapse rates in place around 00z. Main concern is that an enhanced
band of snow could impact the I-70 corridor late this afternoon.

Along the I-25 corridor/northeast plains, a cold front will push 
north to south around 00z. The surface winds appear to be 
north/northwesterly with fairly strong winds over the plains late 
this evening with strengthening cyclogenesis over southwestern KS 
occurs.  The best chance of snow will likely occur along the frontal 
boundary as it interacts with the increasing mid and upper level 
support of the trough and the upper level jet.  Models solutions 
appear to keep the heaviest snowfall potential south and southeast 
of Denver over the Palmer Divide, from 8 pm to midnight tonight. 
SREF plumes generally show snowfall from 0.5-1 inch over Denver, 
with a band of 2-5 inches potentially developing to the south. 
This area could warrant a Winter Weather Advisory as well for 
zones 36 and 41, but more uncertainty to contend with here. As a 
result, we will let the day shift have the final determination on 
this issue. Conditions will improve from northwest to southeast 
after midnight. 

.LONG TERM...(Sunday through Friday)
Issued at 228 AM MST Sat Feb 20 2021

On Sunday, the trof axis will be shifting east and south of Colorado
with a subsident airmass building in behind it. Expect drying 
conditions across the plains, while mountain locations may still 
see some lingering snow showers with some shallow low level 
moisture and continued  west to northwest orographic flow of 
15-30kt. Decent low level mixing up close to 700mb will result in 
gusty northwest winds Sunday afternoon on the plains with gusts up
to 30 mph, especially east and north of the Interstate 25
Corridor. 

For Sunday night and Monday, there will be a moderate northwest flow 
aloft over Colorado. Generally dry airmass but could see still some 
low level moisture still up along the divide and west slopes and gusty
winds over mountains and foothills. Cross sections showing cross 
barrier flow of 30-40kt along with a critical layer Monday 
morning. Flow not brutally strong but given above parameters and 
decent inversion around 750mb, might expect wind gusts up to 60 
mph in wind prone areas Monday morning. On Monday night, flow 
aloft shifts more westerly and increases at mountain top level 
with wind speeds around 50kt. With the downsloping flow, expect 
Monday and Tuesday will be the warmest days of the week across the
plains with readings in the 50s. 

For the out periods from Tuesday through next weekend, Colorado will 
remain under a northwest flow pattern while several waves expected 
to move through the flow. This will result in periods of snow in the 
mountains with decent orographic flow. Expect systems of various
strength to move through the flow on tuesday night and again on 
Wednesday night and early Thursday. These will be accompanied by a
cold front through the plains and expected cool downs on 
Wednesday and remaining below normal through the end of the week. 
Expect best chances of snow will remain in the mountains and 
adjacent plains. Drier conditions for locations further away from 
the mountains. There are some differences in the model solutions 
for the Wednesday night system as the European model cuts off the 
low near the 4 corners and a bit stronger than the GFS solution. 
For now, will keep cooler temperatures and a chance of snow for
the plains.  

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 228 AM MST Sat Feb 20 2021

Expect VFR conditions at all terminals through 00z, with
deteriorating conditions following the passage of a cold front.
Low clouds and light snow are expected to develop behind a strong
cold front. Southerly winds are expected to persist through this 
morning with speeds of 8-12 kts. The cold front will push through 
the area around 01Z this evening. The winds should then turn to 
the WSW and increase, with moderate gusts in the 20 to 25 kt 
range. Winds will turn NNW and further increase with the frontal 
passage, with gusts 25-35 kts. Some light snow showers in the 
vicinity of the terminals are possible with the frontal passage. 
The best chance of accumulating snowfall is a few hours after the 
frontal passage from 03z-07z, and a trace to 2 inches seems 
likely. There is the potential for an enhanced band of heavier
snow to develop but so far most of the model solutions keep this 
feature south of Denver area. Expect MVFR to IFR conditions with 
snowfall behind the front. Ceiling and visibilities should start 
to improve by early Sunday morning.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 2 PM this afternoon to 5 AM MST 
Sunday for COZ033-034.

Winter Weather Advisory until 5 AM MST Sunday for COZ031.

&&

$$

SHORT TERM...Cooper
LONG TERM...Entrekin
AVIATION...Cooper