Tuesday, February 21, 2012

ECMWF Printing Dicey Situation for East Coast


The ECMWF is printing a potentially dicey situation for the East Coast in a possible severe weather event scenario. What we have here is hour 168 of today's 12z ECMWF. It is considered long range, so don't get too hooked on it.

This storm system looks to be a pretty well organized system, with a warm sector in front of it, pulling up warm, unstable air from the South as shown by the brighter colors being pulled towards the center of the storm. Another side of the storm is the following cold air, shown by the cooler colors in the Great Lakes. As shown in the spring and summer, severe thunderstorms do happen when opposing air masses collide. However, luckily, it is not spring just yet.

Here is the relative humidity forecast for hour 168. Relative humidity (RH) values at the 850mb level, which is shown here, are usually indicative of precipitation at the ground when values are 80 or higher. Let's take a look at the area where the storm may affect people. Much of Canada and the Great Lakes are under a big swath of 90% and greater RH values. This would most likely be the snowier part of the storm system.

Of more interest is the RH values along the East Coast. The way the RH values are positioned in a diagonal line of sorts along the Coast makes me concerned that the ECMWF is looking at a squall line forming. This really would not surprise me, with the warm sector being pulled out ahead of the storm system. The good news is that these RH values are between 60% and 90%, meaning that confidence in this possible squall line is low. The ECMWF does not publicly distribute precipitation charts, so these RH charts are the next best thing.

An area of 90%+ RH values does exist to the east of Pennsylvania. That does appear to be a rain event, judging by the 850mb temperatures and the proximity to the storms center. Again using proximity, it does not look to be a huge severe weather event, but some storms are possible offshore, as this blob of higher RH may be the warm front pulling up the warm air ahead of the storm.

Lastly, we have 850mb wind speeds at hour 168. Meteorologists use this to determine if tornadoes are possible. In areas of higher wind speeds, this tornado risk is increased. Looking at the map, we see widespread wind speeds of 30 knots to 40 knots onshore, which is pretty low in terms of severe weather potential. Using this without looking at other wind parameters which also have a say in possible tornadoes, tornadoes seem pretty unlikely in my opinion.

Offshore may be a different story. The warm sector out ahead of the possible storms combined with maximum wind speeds of 60 knots just offshore the Northeast region tells me that waterspouts are not out of the question. Waterspouts are basically tornadoes on water, but with a lot less strength but still with damage potential. It is somewhat rare to have waterspouts hit land after forming at sea, and even rarer to have those waterspouts continue on land.

All in all, this doesn't look like a huge severe weather event if it does happen. The ECMWF does appear to be hinting at a possible squall line, which would provide some concerns, but again, nothing too major.


6-9 Inches of Snow Possible for South Wisconsin

The 12z NAM is printing out 6 to 9 inches of snow in south central Wisconsin over the next 3 and a half days. This comes as the model projects a storm system to move through Southern Wisconsin and strengthen as it does so, with some additional, lower amounts coming from other small storm systems.

The main system in question will be a clipper system, meaning that little to no Gulf of Mexico moisture will be available to this storm. This provides some benefits and disadvantages. A big benefit includes the lessened risk of a dramatic rain/snow line, which thus cuts down on icing and also leaves more snow than rain. One of the bigger disadvantages, however, is how the strength of the system will be hindered due to the lack of Gulf moisture.

A caveat I am seeing with this storm is the presence of a low pressure area in the Southwest. The jet stream will be positioned to have little influence from the low pressure system. Influence meaning the jet stream being dragged south. And this is where the problems are. I see the potential that the models may be underestimating the jet stream's position, meaning that the jet may be farther south than what is being shown. This development would bring the system south and essentially do the same with snow amounts.

Any questions may be asked below.


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