Wednesday, October 7, 2015

MWN Lightning Round: Joaquin forecasts, 1000-year rain, and the weekend forecast

It's time for another edition of the MWN Lightning Round! Today we tackle a couple topics related to recent events on the east coast and tropics, as well as take another quick look at the weekend forecast, which has changed a bit in the past couple of days.

Hurricane Joaquin track forecasts

Early last week, eyes turned towards the warm waters of the Caribbean as Joaquin spun up and quickly became a major hurricane as it churned into the Bahamas where above average warmth and low wind shear aided rapid development. There's been a lot of talk in the aftermath (when you can break through the conversation about SC flooding, which is definitely worthy of plenty of it!) about the performance of the American (GFS) model vs. the European (ECMWF) model.

Once again, the ECMWF handily beat its American counterpart on the forecast track of Joaquin, indicating that the nearly 10-year long streak of no major hurricane landfalls on U.S. shores would remain intact. While the European model nearly pegged the eventual track of the storm as early as Monday night, the GFS didn't release the east coast from its sights until Friday morning (after the storm had reached the Bahamas) and took another 4 model runs (24 hours) to correctly show that there was a higher threat to Bermuda than the Canadian Maritimes. Credit goes to the National Hurricane Center for keeping the official forecast track off the east coast and not issuing tropical watches on U.S. soil in deference to the European model, despite the consensus of the rest of the models showing an eastern U.S. landfall.

The European model forecast for Hurricane Joaquin as of Monday night, September 28th, correctly showing a trip into the central Bahamas, followed by a turn to the northeast, close approach to Bermuda, and eastward turn south of 40° north. Graphic courtesy WeatherBell Analytics. Used with permission.
The American (GFS) model forecast for Tuesday night, September 29th, showing landfall near Norfolk, VA and significant direct impact to the Mid-Atlantic region. This forecast was made 24 hours after the above track produced by the European model. The GFS didn't eliminate direct impact to the U.S. as a possibility until late in the week. Graphic courtesy WeatherBell Analytics.
Having followed these models regularly for years, my primary comment here is this: the American model generally does a very good job in the short to medium-range (out to a week) and compares favorably with the European model in general. However, when the spotlight is on (Hurricanes Sandy and Joaquin immediately come to mind), it has fallen short. What must not be forgotten is that the massive northeast blizzard that was predicted to bury NYC last winter (and ended up being a "regular snowstorm" for a well-prepared northern metropolis) was actually best forecast by the American-made GFS, whereas the European led forecasters astray, at least in the area that gets the most media attention, the Big Apple.

What is a 1,000-year rain event?

Since we're talking about Joaquin, this is a good opportunity to discuss another misunderstood, and unfortunately poorly-explained, topic (at least by the mainstream media) that we've all heard in reference to the historic flooding this past weekend. Portions of South Carolina received a 1,000-year rain event. What does that mean, besides that it is historic, catastrophic, and devastating all at the same time?

Let's start with what it is NOT. First, it was not a 1,000-year FLOOD event. That is different from a 1,000-year RAIN event. A 1,000-year rain event does NOT mean that: a) it's been 1,000 years since the last rain of this magnitude, despite what Stephen Colbert or the Governor of South Carolina says, b) it'll be 1,000 years before this much rain falls again, or c) it only rains this much every 1,000 years.

A 1,000-year rain event actually means that there is a 1/1,000 chance that a certain amount of rain will fall in the given period, or that there is a 0.1% chance of it occurring. Could it occur again next year? Sure! A good way to describe this (and credit to the CoCoRaHS blog for this great example) is to imagine 1,000 ping pong balls in a box. All but one are white, the other is red. With your eyes closed, you pull a random ball out of the box. You have a 1/1,000 (0.1%) chance of grabbing that red ball. You then put that ball back in the box, mix them up, and grab a ball again. Once again, you have a 0.1% chance of pulling the red ball. The fact that you picked it out the first time does NOT affect your chances the subsequent time. Therefore, a 1,000 year rain event may not happen for 2,000 years, or you may see it much more often than every 1,000 years!

The last comment on these "recurrence interval" events, whether 1,000-year, 500-year, or 100-year event, is that the amounts of rain vary significantly from one place to another. For example, for Memphis, a 1,000 year rain event is 10.64" in 24 hours, while in Phoenix, it is 4.82" in 24 hours. Also, average recurrence intervals are calculated not just on 24 hour rainfall amounts, but on several different timeframes. The recurrence intervals for Memphis are shown below. As an example, a 100-year rain event is 8.02" in 24 hours or 3.20" in 1 hour.

Average recurrence intervals for precipitation events for Memphis, TN. Data courtesy City of Memphis/Shelby County Storm Water Management Manual.

Weekend forecast

In a post earlier this week, we discussed the warm weather mid-week (which we're experiencing... mid 80s can leave anytime thank you) and the passage of a cold front on Friday. That post mentioned minimal rain chances with the front, but computer models are picking up on more moisture along and behind the front and we've added a 30% chance of showers and maybe a few thunderstorms to the Friday afternoon forecast, as well as a 40-50% chance of evening showers.

GFS Wednesday morning forecast model of total rainfall for Friday and Friday night. Graphic courtesy WeatherBell Analytics.
The good news: we could really use some rain after a dry month, so a quarter inch of liquid will be welcome for some, plus it'll be gone for the Columbus Day weekend, so expect cooler and drier conditions Saturday and Sunday with clouds departing by mid-day Saturday. The bad news: you might need a poncho or umbrella at any Friday night events outdoors, including football games and the Levitt Shell concert. If forecast trends hold, we could see a steady rain during the evening with north breezes making it feel a fair amount cooler than the daytime hours when highs will reach the lower to mid 80s.

Keep abreast of the latest forecast conditions from MWN with our mobile apps, perfect for checking radar and our human-powered forecast while out and about. Links to download can be found below.

Erik Proseus
MWN Meteorologist

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Monday, October 5, 2015

Warmer weather returns, but the next front is on the horizon

For those who didn't like the cool fall weather over the weekend - whether it was because it was too early in the year or because no sun accompanied the chilly temps - you're in luck. Temperatures are climbing back to above normal readings for the rest of the week with a taste of late summer returning in the form of mid (and maybe even a few upper) 80s highs.

You can thank the departure of a large upper level low from the southeast U.S. (one that funneled tremendous moisture from Hurricane Joaquin into the Carolinas) and building high pressure for the climbing temps. In fact, much of the nation will be under upper level ridging between the lows over the Atlantic (one off the SE coast and one associated with Joaquin) and another low in the southwestern U.S. Fair weather, with a mix of clouds and sun, will dominate the rest of the week as those temps climb under the upper level ridge.

The upper level (35,000') map for this evening shows ridging over the Mid-South between low pressure in the Desert Southwest and over the east coast. 

As we head towards week's end, a front will sink south into the region. However, little upper level support accompanies it, so though clouds are expected, rain chances appear fairly minimal. Bad news for those who, even at this time of year, depend on some precipitation and haven't gotten much of it the past 30 days or so. Any rain chances would likely be highest from Friday to early Saturday.

Forecast rainfall for Thursday night through Saturday indicates very little precipitation expected across the Mid-South.
Following the front, somewhat cooler weather arrives again for the weekend, but only briefly as a ridge of high pressure re-builds and pushes temps back above normal for next week. The long-range outlook from NOAA for next week indicates a high likelihood of above average temperatures for a large portion of the nation.

NOAA's temperature probability map for week 2 (Oct. 13-19) shows a high probability of above average temps for most of the nation.
You can always get the latest forecast information, current conditions, radar, satellite, and severe weather outlooks via the MWN mobile apps for Android and iOS. Also be sure to check out our StormWatch+ severe weather notification service within the app!

Erik Proseus
MWN Meteorologist

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Saturday, October 3, 2015

September 2015 Climate Data and Forecast Accuracy

September Recap

September typically begins the welcome decline in average temperatures and is one of the driest months of the year. This month definitely lived up to the usually dry pattern, but temperatures were slow to depart from summertime trends. Temperatures ended the month nearly 2.5 degrees above normal.

Precipitation-wise, all but 0.20" of the rain that fell occurred on one day. Over the past two months, the vast majority of precipitation has fallen on just a few days, leading to "abnormally dry" conditions appearing in the Horseshoe Lake and Tunica County portions of the metro, according to the U.S. Drought Monitor. Through September, Memphis International Airport's annual rainfall deficit is 3.18", or about 92% of normal.

There was no severe weather in the Memphis metro during the month of September. In fact, no thunderstorms were reported at the airport for the entire month.

Memphis International Airport, Memphis, TN

Average temperature: 77.5 degrees (2.3 degrees above average)
Average high temperature: 87.5 degrees (2.4 degrees above average)
Average low temperature: 67.5 degrees (2.3 degrees above average)
Warmest temperature: 97 degrees (5th)
Coolest temperature: 53 degrees (13th)
Records set or tied: None
Comments: None

Monthly total: 1.32" (1.77" below average)
Days with measurable precipitation: 5 (2.3 days below normal)
Wettest 24-hour period: 1.12" (9th)
Total Snowfall: None
Records set or tied: None
Comments: A dry month was made even drier by the fact that 85% of the total monthly precipitation fell on one day.

Peak wind: Northwest/32 mph (11th)
Average wind: 5.7 mph
Average relative humidity: 63%
Average sky cover: 40%

Click here for a daily statistical recap for Memphis International Airport.

Cirrus Weather Solutions /, Bartlett, TN

Average temperature: 73.5 degrees
Average high temperature: 86.7 degrees
Average low temperature: 62.4 degrees
Warmest temperature: 96.5 degrees (5th)
Coolest temperature: 47.6 degrees (13th)
Comments: None

Monthly total: 1.43" (automated rain gauge), 1.49" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 5
Wettest date: 1.00" (9th) (via automated gauge)
Total Snowfall: None
Comments: None

Peak wind: Northwest/18 mph (12th)
Average relative humidity: 79%

Click here for a daily statistical recap for Bartlett, TN.

MWN Forecast Accuracy

MWN average temperature error: 1.48 degrees
MWN forecast temperatures within 2 degrees of actual: 85%
MWN average dewpoint error: 1.70 degrees
MWN forecast dewpoints within 2 degrees of actual: 77%

MWN's forecasts extend out five periods (2.5 days, or roughly 60 hours). Historical accuracy statistics can be found here.

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Upper levels producing flooding in South Carolina and record cool temps locally

An upper-level weather pattern that is fairly unusual is combining with the strongest hurricane of the year to produce varying rare conditions locally and across the Southeast.

Starting outside the Mid-South, Hurricane Joaquin has gotten even stronger today as it departs the rain and wind-lashed Bahamas, heading on a northeast track that will take it well offshore the U.S. and side-swiping Bermuda.

Major Hurricane Joaquin with 155 mph wind departs the Bahamas heading northeast into the western Atlantic.
Just a notch below category 5 status at 155 mph sustained wind, the hurricane is not having a direct impact on U.S. weather. However, indirectly it is contributing to a potentially catastrophic flooding event, as described by NOAA, over parts of the southeast U.S., most directly in the Carolinas and northeast Georgia. As shown in the graphic below, a river of water-logged air extends from Joaquin into SC, which is feeding persistent heavy rain, dropping multiple inches per hour in some cases!

Precipitable water (PW) values of 2"+ (red) are clearly seen being drawn into SC from Joaquin this afternoon, creating flooding rainfall. Graphic courtesy WxBell.
Portions of South Carolina are experiencing record rainfall and possible catastrophic flooding along the lines of a 500-1,000-year event, according to the Weather Prediction Center.
Areas like Charleston, SC are receiving record rainfall that are being attributed to Joaquin by some, but really is the result of a deep upper-level low pressure system over the southeast U.S. (shown below) that is helping to draw Joaquin's moisture into the region.  That same upper low is also responsible for our overcast and chilly conditions in the Memphis metro.

A large upper level low over the southeastern U.S. is drawing abundant moisture from Joaquin (lower right), as well as creating cloudy, cool conditions over the Mid-South. Graphic courtesy WxBell.
On the backside of the low, in the Mid-South, persistent northerly flow around the low and moisture wrapping around said low has created cloudy skies the past couple of days, keeping temperatures well below normal. In fact, today's high temperature will be a record cool max topping out (so far) at 59° in the wee hours of this morning. Add a north wind at 10-15 mph and sweatshirts and jackets have been retrieved from the closet as fall has arrived!

Early afternoon visible satellite imagery shows persistent cloud cover over all of the southeastern U.S., including the Memphis metro.
Over the next couple of days, the southeast upper low will weaken a bit and move east as Joaquin moves out, allowing for some sunshine to re-appear in the Mid-South sky. A little is possible tomorrow afternoon, which will help temperatures warm back to the mid 60s to near 70 depending on how long how solar radiation we receive.

By mid-day Monday, the upper low will have shifted offshore and weakened slightly, allowing high pressure to build into the Mid-South, warming temperatures and bringing more sunshine. Graphic courtesy WxBell.
By Monday, we'll be back near 80 with more sunshine expected. Conditions remain dry into next week as temperatures warm back into the mid 80s for much of the week, a result of high pressure at the lower and upper levels taking hold. The next chance of rain doesn't appear until late in the week as the next frontal system approaches. Keep track of the latest forecast on our mobile website or in the MWN mobile apps (link below).

Erik Proseus
MWN Meteorologist

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