Day 76: HPC Snow Forecast
Tonight I continue discussing the potential weekend storm, in particular the possibility of winter weather. The image above is the Hydrometeorological Prediction Center’s probability of a point exceeding 4″ of snow accumulation between 7 PM CDT Friday and 7 PM CDT Saturday. Notice how a large potion of Oklahoma is contoured within the 10% threshold and areas from Oklahoma City northeast along I44 and west back to I35 are contoured in the 40% threshold with areas in northeast Oklahoma along the Kansas border contoured in the 70% threshold!
Below is the same kind of graphic, for the same time period, only instead of the probability of a point exceeding 4″ of snow accumulation, this is for 8″ of snow. Again, northeast Oklahoma is the area targeted.
This time the plot below is for 12″ snow accumulation. While the probabilities are low, areas are still being highlighted.
Now, without going into all the details of the the data I’ve examined up to now, I am beginning to think this storm won’t be as big of a deal as I was thinking 2 days ago. The reason for this is that the storm is moving relatively quickly, moisture isn’t all that great, and most of the precipitation in the last few model runs has been tied to the frontal zone – which moves fairly quickly through Oklahoma. Based on this, I think the graphics posted above are a bit optimistic regarding Oklahoma’s snow chances. However, the event is still 2-3 days into the future, so a lot can change between now and then.
Day 75: ECMWF 96-120 Hour Forecast
As I mentioned last night, big changes are looming on the horizon for people in the southern plains of the United States. Tonight I’ll examine two different forecast fields for two different forecast times as simulated by the European Center for Medium-Range Weather Forecasting (ECMWF) numerical model, run at 12 UTC this morning (Tuesday). The first image is valid 12 UTC Saturday (7 AM CDT) and the second image is valid 12 UTC (7 AM CDT) Sunday.
One of the things that should have been conveyed in last night’s post is that there exists a potential for snow across portions of the southern plains between Friday night and Sunday morning. One of the things that a meteorologist will examine when forecasting for snow is the temperature of the 850 mb pressure level (roughly 1.5 km above ground level over the ocean). If this level is above 0C, or 32F, then snow is extremely difficult to get at the surface, however other winter precipitation types (sleet or freezing rain) would be possible if the surface temperature is below 0C, or 32F. If this level is below 0C, or 32F, then snow is possible, assuming precipitation did fall.
The image above is the forecast 850 mb temperatures for 12 UTC Saturday, followed by the 12 UTC Sunday 850 mb temperatures. As one can see, temperatures across a good potion of Kansas, and the western 2/3 of Oklahoma, are below 0C (32F) by 12 UTC Saturday (7 AM CDT). This would mean that if precipitation were to occur, snow would be one of the possible precipitation types about which to worry. By 12 UTC Sunday (7 AM CDT), the 850 mb temperature (shown below) has cooled even further across an even larger area – including much of Arkansas.
This, in and of itself, would certainly suggest that meteorologists would need to be concerned about the potential for snow, should precipitation occur. However, I haven’t said anything about whether precipitation (and, in particular, snow) would be falling at the two times discussed above. The plot immediately above and below attempt to address the, “Will is precipitate?” question. These two images are the 700 mb relative humidity fields – which are being used as a proxy for precipitation fields, since those are not freely available.
Most snowflakes (the big, pretty ones anyways) tend to develop around 700 mb in what is known as the dendritic growth zone, if sufficient moisture can be found at this level. Based on this, if the relative humidity is high at the 700 mb layer, we can (to a first approximation) assume that snowflakes are certainly possible in the cloud. If we couple that with the temperatures at 850 mb being below 0C (32F) we can make a first guess forecast as to whether snowflakes are possible to reach the surface.
If we notice the 12 UTC (7 AM CDT) Saturday forecast, we see that the relative humidity is extremely low at 700 mb across central Oklahoma. This is known as the dry slot, and is a good indicator that precipitation would not be occurring at the surface. However, notice the high relative humidity values to the south and west of central Oklahoma at this time. By 12 UTC (7 AM CDT) Sunday, both of these features have moved east of Oklahoma. Since I do not have forecast images from 12 UTC Saturday and 12 UTC Sunday, we are left to assume that the positions of the Saturday morning features have translated to their new locations Sunday morning. (This far into the future, that is a relatively safe approximation.) If we do this, we can assume that the moist air at 700 mb will traverse the state of Oklahoma, with 850 mb temperatures (first two plots above) below 0C (32F), during the day Saturday. This would indicate that snow is a possibility (but not a certainty) during the day Saturday throughout central Oklahoma. It will certainly be interesting to watch this unfold during the next few days…
Day 74: Upcoming Pattern Change
I normally do not include graphics such as those above in my 365 Blog, however, I’ll make an exception tonight. These graphics, generated by the National Weather Service Forecast Office in Norman, OK, adequately capture my growing concerns for late this week into next week. The exact details are still in question, but a strong storm moving through the central plains will tap into Gulf of Mexico’s moisture and the arctic’s cold air. Before the cold air sweeps into the southern plains, severe thunderstorms will be possible in the warm sector of the storm, followed by a chance of accumulating snows in much of Kansas and possibly as far south as Oklahoma in the cold sector.
A lot can, and most likely, will, change in the coming days. This developing high-impact weather event will most likely be the focus of my blogs this week.
Day 73: Warm(er) Oklahoma Evening
Anyone outside last night across Oklahoma may have noticed a slight chill in the air compared to temperatures from earlier in the week. Tonight, however, temperatures are running anywhere from 5-15F throughout a large part of Oklahoma – in fact, some areas of southeast Oklahoma are almost 20F warmer tonight! The warmer temperatures should last a few more days before actually increasing even more before next weekend.
Late in the week, into next weekend, models have hinted at the possibility of a pattern change back to colder temperatures. I’m sure this will be the focus of blog posts in the coming days…
Day 72: Strong Nor’easter
An extremely strong nor’easter is currently ongoing along the east coast of the United States. Winds are gusting around (and above) hurricane force on Long Island, NY this evening (64 kts or 74 mph at 7 PM EST at JFK). I’ve heard anecdotal stories of over 100,000 customers without power with many reports of transformers blowing. As you can imagine, trees have been blown down and roofs have been damaged!
The storm surge associated with this event is also impressive, as it is the highest since the 1996 nor ‘easter and close to the 1992 event that actually flooded portions of the NYC subway. Examining the image above, you can see the surface low pressure (around 995 mb) and a relatively strong high-pressure in southeast Canada (greater than 1028 mb). The position of these two is resulting in a very strong pressure gradient and extremely strong winds. Also, the orientation of the gradient (surface low still on shore in the Mid-Atlantic region) is allowing the strong winds to travel over a large portion of the Atlantic Ocean – resulting in very large waves and piling of water into New York Harbor. This is a classic example of coastal flooding.
In the image above, the white lines are the actual surface pressure values whereas the blue and red lines indicate surface pressure rises and falls, respectively. Notice that the greatest pressure falls are now centered south of Massachusetts. This would lead me to believe that the strongest wind event of the night is over for New York City, NY and will slowly spread northeast along the coast of southern New England. I’ll be interested to hear what kind of winds occur in the Boston, MA area overnight.
Day 71: East Coast Flooding
The storm that brought severe weather to the central United States this week is continuing to move east. To the east of the storm, warm air is being drawn northward over areas that have received a lot of snow this winter. As a result, rapid snow melt and, on top of that, rain, is occurring across portions of the Mid-Atlantic and southern New England.
In the image above, areas throughout the Mid-Atlantic and southern New England (shaded in various forms of green) are under some form of flood watch, warning, or advisory. In the coming weeks, areas across the northern plains will be experiencing this a lot as well.
Day 70: …And What Happened Yesterday
In last night’s post, I displayed a screen capture from the radar display I use to monitor severe weather events as they unfold throughout the United States. It was a very active severe weather evening across much of Arkansas – especially the SW to NE corridor through Little Rock. The image above is the preliminary list of all severe weather reports that the National Weather Service as received. As you can see, Arkansas had just about every kind of weather that one could expect from severe thunderstorms. In fact, I’m sad to report that one of the tornadoes resulted in a fatality in Arkansas. You can read more about it here.
About Patrick
Patrick Marsh is a Ph.D. candidate at the University of Oklahoma's School of Meteorology. He earned a Bachelors of Science in Mathematics and Physics (cum laude) from the University of Arkansas in 2005 and a Masters of Science in Meteorology from the University of Oklahoma in 2007. In addition to working on his doctorate, Patrick serves as the National Severe Storms Laboratory Liaison to the Hazardous Weather Testbed where his research interests lie in working with operational forecasters, data mining, data visualization, and all things high-impact weather. For more about Patrick's work, check out his complete CV!
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