textproduct: Bismarck

KEY MESSAGES

- Patchy fog across central North Dakota this morning, dense at times near the Turtle Mountains.

- 20 to 40 percent chance of light freezing rain moving from west to east across the state late this evening through Friday afternoon.

- Above normal temperatures continue into the weekend, with highs mostly in the mid 20s to lower 30s north and east, and lower to mid 40s southwest.

- Next week will be much colder and mostly dry, with only low to medium chances for light snow across the south on Monday.

DISCUSSION

Issued at 351 AM CST Thu Dec 26 2024

A meridional surface trough is analyzed over central North Dakota early this morning, with a low-amplitude upper level trough entering western North Dakota. The only concern for this morning is patchy fog across central North Dakota, which has become periodically dense near the Turtle Mountains and to a lesser extent near Bismarck. The coverage of fog is much lower than previous nights though. Both the surface and upper trough are forecast to wash out over the state today, leading to benign weather through the afternoon. The forecast for today features a partly cloudy sky, light southerly winds around 5 to 10 mph, and high temperatures ranging from the mid 20s north central to mid 40s southwest.

A mid to upper level shortwave is forecast to glide across the Northern Plains later this evening through Friday afternoon. Many aspects of the precipitation forecast with this wave remain uncertain. First, there is uncertainty in the strength and orientation of the shortwave. The 06Z NAM projects a negatively tilted trough that morphs into a stacked low over southwest Manitoba, while other solutions keep a deepening upper low farther to the north and west. The NAM solution is not represented in 00Z global ensemble clusters, but there are two distinct camps within the clusters for the positioning of the upper low by Friday evening, with around 55 percent membership over southern Saskatchewan and 45 percent farther northwest along the Alberta/Saskatchewan border. The 45 percent outcome features higher probabilities of measurable precipitation than the 55 percent outcome, but the overall probability thereof from all global ensembles is around 20 to 30 percent in central North Dakota, 30 to 50 percent in eastern North Dakota, and 40 to 60 percent in western North Dakota. Furthermore, smoothed HREF probabilities for measurable precipitation are generally 10 to 20 percent higher at all locations. We are therefore maintaining at least a 20 percent chance of precipitation migrating from west to east across western and central North Dakota late this evening through Friday afternoon, and chances in western North Dakota tonight were also raised as high as 45 percent. Possible factors that may prevent measurable precipitation with this wave include a low level dry layer, a lack of low level forcing, and possibly disjointed mid to upper level forcing.

The other major question is precipitation type. Global ensembles maintain high confidence in above freezing 850 mb temperatures, but in the GEFS it has lowered to only a 60 percent chance through Friday morning. The HREF also now projects about a 70 percent chance of above freezing temperatures at 850 mb. This uncertainty is not only being driven by whether wet-bulb cooling will occur, but the properties of the whole thermal profile as some models (e.g., the HRRR) distinctly keep maximum wet-bulb temperatures aloft above freezing, at least through the morning. Taking all this into consideration, our strategy was to keep either rain or freezing rain (determined by forecast near-surface air temperatures) as a dominant type throughout, and a allow a mix of snow/sleet where CAMs are implying a potential for locally higher precipitation rates, mainly in southwest North Dakota tonight and the James River Valley Friday afternoon. Maximum QPF is projected to be very light, with no greater than a 20 percent chance of exceeding 0.05" at any location. If most of this falls as freezing rain though, which does appear to be the most likely outcome, any amount would cause concerns.

Trailing the threat of mixed precipitation, seasonably mild and dry weather is expected through the weekend. Southwest North Dakota has forecast highs in the 40s Friday through Sunday, while northern areas cool from the lower and mid 30s on Friday to the 20s on Sunday. This is in response to the deepening Canadian cyclone pushing colder air farther south. Ensemble guidance is mostly aligned on a very quick-moving shortwave landing on the northern California/Oregon coast on Sunday and racing into the Central Plains by Monday morning. Q-vector convergence and mid level frontogenesis to the north of the wave could bring accumulating snow as far north as southwest North Dakota Sunday night through Monday. Probabilities for at least an inch of snow (using a 10:1) in global ensembles ranges from around 40 percent in the southwest corner of the state to near zero from Williston to Bismarck. The NBM maintains a mostly dry forecast beyond Monday night, but a persistent northwest cyclonic flow pattern over the Northern Plains throughout next week seems susceptible to periods of flurries.

The main story for next week is temperatures falling back below normal. Ensembles are in strong agreement in a cooling trend through at least New Year's Day, with the NBM bottoming out temperatures on Thursday with single digit above zero highs and below zero lows across the north. Southwest North Dakota looks to remain comparatively mild though, with highs in the 20s and lows above zero. There is still some uncertainty in the degree of coldness next week, and it is interesting that the coldest ensemble clusters contain virtually no ECMWF members. There is also uncertainty on whether an appreciable warmup will occur after Thursday, with extended range guidance showing stark contrasts between cold (GEFS) and warm (ECMWF ensemble) anomalies through the first full week of January.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1147 PM CST Wed Dec 25 2024

Generally VFR conditions will return across the forecast area tonight. Some central and eastern sites such as KBIS and KJMS may still see patchy fog develop tonight and linger into Thursday morning. Confidence was not high enough to place fog in the TAFs at this time, although have placed in TEMPO groups for lower vis and BR. Most of the day Thursday then looks to see VFR conditions with light westerly winds becoming a light southerly wind. Thursday evening, cloud cover looks to increase from west to east. Rain and freezing rain could return to western ND by the end of this TAF period. For now have placed PROB30 group in the KXWA TAF, although these chances could also approach the KDIK area. MVFR conditions will be possible with these showers.

BIS WATCHES/WARNINGS/ADVISORIES

None.