Chilly Tonight and Saturday:
That large area of surface high pressure will continue to drift southeast, with winds slowly diminishing and shifting to a more northerly direction. Bitterly cold temperatures and wind chills will continue with lows tonight falling into the -2 to 5 degree range.
Even though winds will be diminishing, we will hold onto light northerly winds overnight and into Saturday morning. This will drop wind chills into the -9 to -19 range. The current Wind Chill Advisory looks good with an expiration time of noon on Saturday. Temperatures on Saturday will have a hard time warming as that high pressure settles into the Mid-Mississippi Valley. The temperatures are tightly clustered around highs in the middle teens along the Highway 54 corridor to the lower twenties near the Arkansas border.
Saturday Night into Sunday:
The heart of that surface high pressure will continue to slide southeast and into the Ohio Valley. Saturday night will again be very cold, especially across the eastern Ozarks, where winds will remain light. Lows in the single digits are strongly supported by statistical data roughly along and east of a Warsaw to Ava line. Increasing southerly winds across western Missouri and southeastern Kansas will keep temperatures "warmer," with lows generally falling into the lower teens.
Wind chills across most of the Missouri Ozarks will again fall below zero. While wind chills may approach -10 degrees across the eastern Ozarks, 12Z HREF probabilities for wind chills reaching this threshold are less than 10%. Thus, there is no consideration for another Wind Chill Advisory at this point.
Ensembles are in good agreement regarding the synoptic scale pattern. A weak upper-level ridge will translate east across the area on Sunday, resulting in a dry day. Clouds will be on the increase as an upper-level trough associated with the southern jet stream slides east across the Four Corners region. Despite the expected increase in cloud cover, Spreads are relatively low for high temperatures. Areas west of Highway 65 will warm into the lower to middle 30s. The eastern Missouri Ozarks will only warm into the upper 20s.
Freezing Rain Potential Late Sunday Night into Monday Night:
Ensembles are in good agreement that the upper-level trough will remain positioned over the southern Rockies. This will put the Missouri Ozarks beneath a southwesterly flow aloft. Multiple shortwave troughs are forecast to eject out across the central
Plains, one on Sunday and another on Monday night.
A low-level jet is forecast to develop and strengthen from Sunday night into Monday. GFS ensemble integrated vapor transport imagery reveals a mid-grade atmospheric river event, with that jet coming off of the western Gulf of Mexico. Thus, warm air and moisture advection will be strong from the southern Plains and Lower Mississippi Valley northward into the central Plains and Missouri Ozarks.
We currently have precipitation chances running in the 20-60% range for late Sunday night, with the highest PoPs across southwest Missouri. This is much higher than the deterministic which only has PoPs in the 10-30% range. High probabilities for precipitation (80-100%) will then persist into Monday and Monday evening as that short wave trough moves across the region and warm air advection continues above the surface.
Our confidence continues to increase that precipitation will initially start as freezing rain across much of the area. Statistical data from 100 global ensemble members is quite
telling. Probabilities for 850 mb temperatures being greater than 3C (proxy for a warm nose being present) are above 90% south of a Nevada to Rolla line by 12Z Monday. This gives us very high confidence that hydrometeors will fully melt before reaching the surface.
There are then two factors that are particularly concerning with the late Sunday night/Monday morning scenario. First off, wet bulb temperatures will be well down into the 20s as precipitation starts. This gives us very high confidence that liquid hydrometeors will freeze on contact with exposed surfaces (freezing rain). Of perhaps greater concern is that the ground remains frozen across the region. We are currently carrying a three inch frost depth here in Springfield. This is strongly supportive of ice accretion on elevated surfaces and roadways. We are becoming increasingly concerned that road conditions will rapidly deteriorate and become hazardous late Sunday night and Monday morning, including the Monday morning commute.
We do expect freezing rain to gradually change to rain from west to east across the Missouri Ozarks on Monday and Monday evening. As we have seen many times in the past, near or sub- freezing temperatures may hang on quite awhile across the eastern Ozarks. If the colder scenario materializes, freezing rain will persist in some areas of the eastern Ozarks into Monday night.
The experimental probabilistic Winter Storm Severity Index continues to paint 60-80% probabilities for at least minor road impacts, especially along and south of the I-44 corridor. Probabilities for moderate impacts are running in the 30-50% range across the same area.
It is still a bit soon to be advertising ice accumulation amounts given uncertainties with several key components for determining ice accretion potential. Ultimately, precipitation
rates, wet bulb temperatures, and wind speeds will all have a play in eventual ice amounts. The Freezing Rain Accumulation Model (FRAM) takes all of these factors into consideration.
Initially, ice-to-liquid ratios will start out around 1:1 if precipitation comes in fairly light. Heavier precipitation rates would decrease that ratio. As wet bulb temperatures warm
to near the freezing mark, the ratios will like drop closer to 0.6 to 1. In other words, 0.10" of liquid equivalent would yield 0.06" of flat ice.
Overall, this system does not fit the climatology of a classic ice storm across the Missouri Ozarks (ice accretion greater than 0.25"). However, we do suspect that a few areas along the higher terrain of the Ozark Plateau could flirt with this amount, especially if QPF amounts end up being closer to the 75th percentile of the NBM statistical data and/or ice-to-liquid ratios stay closer to a 1:1 ratio for a longer duration.
Ice amounts around 0.25" typically do not result in power outages unless there are strong winds (which we are not expecting). If ice amounts sneak up closer to 0.40", that would open the door for isolated power outages. At this time, probabilities for 0.40" of flat ice accretion are in the 10-30% range along the higher terrain of the Ozark Plateau (Sparta to Ava to Mountain Grove to Mountain View).
One final word of caution regarding ice accumulations: there are a lot of websites with "ice accumulation amounts" that do not use the FRAM equation. They simply convert liquid equivalents that are forecast to fall as freezing rain to ice
accretion using a 1:1 ratio. This often results in over-inflation of forecast ice amounts.
Unsettled into Midweek:
The upper level southwesterly flow will persist through midweek before the main trough finally pulls out into the central U.S. either Wednesday or Thursday. Periods of rain will therefore persist through Thursday.
Temperatures will be milder from Tuesday through Thursday as we remain within a general warm air advection regime. The good news here is that temperatures will be warm enough to preclude frozen precipitation.
There are high spreads in NBM statistical data, so it is tough to nail down exact highs and lows. Generally speaking, high temperatures will be in the 40s with lows in the middle 30s to lower 40s. Some areas of southern Missouri may push the 50 degree mark if we warm closer to the 75th percentile.
Source: NWS Springfield