I’m feeling a bit more confident in the overall general forecast today. NAM and GFS both indicate a rain event for Tuesday and most of Tuesday evening. Boundary layer temps are simply too warm for wintry precip despite being in the more synoptically favored area for wintry weather.
Precip will begin to mix with and change to snow around or shortly after midnight TUE/WED. There are still a few questions that linger. GFS continues to indicate a closed UL low that moves right over the TN Valley during this time frame. NAM on the other hand closes off the heights slightly south of the region. The concern with this solution is the threat for dynamic cooling if the GFS does verify. If dynamic cooling takes place the precip would change over sooner and faster. So, with the placement of the UL feature still in question… there lies the majority of the uncertainty.
Regardless, cold air will advect into the region causing a slower transition. There will be a window of opportunity to add up some snowfall accumulation Wednesday morning; however, this will be as precip ends. I think a 1-3” accumulation range is appropriate for the set up. 1” if NAM verifies (with a much slower transition) and 3” if dynamic cooling takes place and a sooner transition.
Forecast Tuesday Night and Wednesday:
Rain changing to snow around or slightly after midnight. Snowfall accumulation of 1-3” is possible for the Sequatchie Valley before precipitation ends on Wednesday. Travel may become hazardous as temps fall to near freezing Wednesday morning.
Meteorologist: Robert Frye
Updated: 1/24/2011 at 4:45 PM CST
As expected, the models are doing a nice flip flop from one run to the next. The biggest concern right now is surface temperatures. We will remain on the north/northwest part of the storm (coolest area), however, models have been trending warmer and warmer. Based on this, I am very skeptical about the upcoming forecast. Another interesting entity to note is the fact that models are delaying the arrival of the storm. With that, I have decided to push the main event back a little. Looks like a Tuesday and Wednesday event now.
Looking at the 12z NAM, the low track has shifted from yesterday’s run. NAM now takes the system close to the GFS/ECMWF solution. Overall, the track position forecast is not in question… it’s temp profiles that worry me. After all, we can’t get frozen precip if temps are well above freezing between 850-surface. NAM shows 850 0°C line well north of us for most of the event duration and finally moves it south right when precip comes to an end.
The 12z GFS does give you snow lovers some hope. It gives me an incredible headache because it only adds to the heap of uncertainty. GFS forecast soundings show ample moisture; however, like the NAM, it does keep the bottom 15%-20% of the atmosphere above freezing. One entity this run of the GFS that really stands out to me is the closing off of heights at 500 very close to our region. If that occurred, one would have to wonder about possibilities of dynamic cooling. If dynamic cooling occurs while the heaviest precip lingers over the area, significant winter weather would become a bigger problem.
As the low moves up the east coast and intensifies, a stronger northerly flow will kick in and bring the colder air into the region regardless of dynamic cooling. So, two things could happen with this… 1) The storm intensifies earlier than expected and brings in colder air a bit sooner… or 2) the storm strengthens as currently suggested leaving only a small window for wintry precip as precip ends.
So many questions and not enough answers right now. I’m going to keep mentioning a threat for snow, but this is a classic border line event that we’ve seen time and time again in previous years. Only a few degrees will mean a world of change.
Forecast for Monday:
A slight chance of light rain. Precip may start off as a mix of frozen precip before changing to all liquid.
Forecast for Tuesday-Wednesday:
Rain will continue to develop during the day Tuesday. Eventually, colder air will rush into the region changing rain over to snow before the precipitation ends Wednesday Morning.
Meteorologist: Robert Frye
Updated: 1/23/11 at 11:00 AM CST
An interesting forecast is getting more and more interesting as time passes. Latest GFS/NAM (12z)/ECMWF (00z) trends indicate that we have the potential yet again to see significant winter weather across the Sequatchie Valley. However, with that said, there are notable differences among computer models. The GFS tracks the low that is expected to develop over the gulf a little further south and keeps us a tad cooler. Snow depth forecasts do indicate significant snowfall accumulation with the highest amounts over the southern portion of the valley/plateau. NAM is more aggressive in terms of moisture; however, it’s a bit too warm for frozen precip in valley locations. NAM tracks the surface low over central Georgia where the GFS takes the low over northern Florida and southern Georgia. Due to this disagreement among models… confidence in the forecast is still low.
I am fairly confident in Monday’s forecast. It appears that a majority of precip Monday will fall as liquid (though frozen precip may be possible at the onset of precip). As the system moves southeast of us on Tuesday, a northerly flow will cause temps to fall. NAM is still a bit out of range so again… will take a hurry up and wait approach. Since the models are misbehaving I am obviously not comfortable with accumulation forecasts. Just a few degrees temp wise can greatly change to overall situation. I decided to go with rain changing to snow Tuesday afternoon as I do expect just enough cold air to make it in before the precip ends. Residents of the Sequatchie Valley are urged to keep updated on the current forecasts as any small changes in the forecast temps and/or low track may lead to significant changes in the general forecast.
Forecast for Monday-Monday Night:
Light rain showers are expected. Some sleet or snow may mix in at the onset; however, precipitation should change over to all liquid later in the day.
Rain will develop and move into the region Tuesday. As cold air rushes into the Sequatchie Valley/Cumberland Plateau precipitation should mix with and change to snow. The majority of precipitation should come to an end Tuesday evening; however, flurries or a stray snow shower will be possible overnight. Hazardous road conditions may develop Tuesday Night as temperatures fall below freezing.
Again, check back for the latest.
Meteorologist: Robert Frye
Updated: 1/22/2011 at 12:00 PM CST