Jun 4, 2013

Heavy Rain expected end of week - possible flooding

Our nice  weather will last through Wednesday the 5th of June. The high pressure system responsible for the nice and cool weather (might see some frost tonight in a few places across the 'Dacks) will slip to our east tomorrow and stall just off the East Coast. The clockwise windflow around the high will cause a very moist onshore SE-S  flow of air from off the Atlantic and from out of the Gulf of Mexico to develop across the region.

The Weather Setup for our end of the week rain

Here is the forecast surface map for Wednesday evening showing the area of high pressure offshore with a complex and broad area of low pressure from the Midwest to the Lower Mississippi Valley-Gulf of Mexico and the Southeast States. Rain is forecast to be widespread in a swath from the ARKLATEX region to the NY/Canadian border.

The Numerical Weather Prediction Data

By Thursday morning 6 June ALL Numerical Weather Prediction (NWP) data (shown below) is showing the broad area of low pressure advancing our way. Note that all of the "models" have an area of low pressure over the Gulf of Mexico. This low is already there currently over the Yucatan Channel  (that's the body of water between Cuba's west tip and Mexico's Yucatan Peninsula). The National Hurricane Center is monitoring this feature for some possible tropical or sub-tropical development as it moves NNE-NE over the next 48 hours Based on this weather data the rain will develop by early Thursday morning across the area.

By Friday morning once again all of the NWPs are in fairly good agreement as to the evolution of this broad area of low pressure (maps below). All indicate the low in the Gulf of Mexico (GMx) moving onshore and across the Southeast States as several areas of low pressure move our way from the Ohio Valley States. The forecast data also indicates that the GMx low strengthens. Whether or not it is a tropical system or not is a moot point as it will likely be a non-tropical low by the time it moves across the SE states due to its interacting with the broad area of low pressure to its west through north.

By Saturday AM (maps below) the coastal low is forecast to be moving across the Northeast States. On this day the models are showing some spread on how fast the low will be moving - the GFS is the fastest with the low over Maine and the UK the slowest with the low over Delaware Bay. Once again all the data does show the low to continue to modestly deepen. This deepening is likely due to baroclinic mechanisms and not tropical.

Secondary and Tertiary Weather Data

Another bit of weather data that I like to use in forecasting precipitation is precipitable water.  Precipitable water (PW) is defined as the depth of water that would be measured at the ground if all of the water vapor in the overlying air was precipitated out. PW varies on both a daily and seasonal basis. Warmer air can have more water vapor in it than cooler air can thus during the late Spring through early fall PW values will be greater than during the other times of the year. Knowing both the "normal" PW for the time of year and what the forecast PW will be is very useful in forecasting both rain fall probability and amounts. Looking at forecast PW values and then comparing the forecast values to normal values one can assess how much moisture is or is not available.

Here is the forecast PW and its standard deviation (SD) from normal from the GFS ensembles (GEFS). The map for Thursday night:
Forecast Precipitable Water and Anomaly for 8PM EDT THU 6 June
PW values of +1 to +2 SD are over NY with +2 to +3 SD amounts across the Mid-Atlantic States (A positive SD in this case means greater than normal or greater than average PW is forecast)

By Friday morning the area of +1 to +2  PW encompasses most of NY and New England while the +2 to +3 PW values extend north from Southeast Pennsylvania  to SE NY State and Southern New England.
Forecast Precipitable Water and Anomaly for 8AM EDT FRI 7 June
Keep in mind that above anomaly maps (and those that follow below) are based off of one particular model - the GFS and its ensembles. If one were to peruse other ensembles for the other models there would/could be differences in the placement and "intensity" (amounts/strength) of the PW/wind.

Lastly forecast low-level wind anomalies for the 850 hPa level (about 5000 feet AGL) show stronger than normal wind strength both supportive of advecting a lot of moisture northwards across NY and New England AND also helping to increase vertical motion or lift.. Here is the forecast 850 wind  and wind anomaly map (from the GEFS) for Friday morning. The top map indicates a -2 to -3 SD anomaly to the 850 hPa wind. The "negative" SD indicates winds have a more easterly component. The second map below shows a very strong positive SD to the southerly component of the 850 hPa.

The S-SE wind also enhances lift across the South and Southeast facing slopes of the Taconics, Greens, Berkshires and Adirondacks; these locations have the risk to have locally higher amounts

I also like to look at other data sets too like surface pressure anomalies, other ensemble data from the various other NWPs, too.  If you are interested here is an excellent report that nicely ties together all of the above that I have posted into a forecast methodology/technique:  THE USE OF ENSEMBLE AND ANOMALY DATA TO ANTICIPATE EXTREME FLOOD EVENTS IN THE NORTHEASTERN U.S.

How Much Rain?

As for rainfall amounts here's the call from HPC for the Wednesday through Sunday period.

My forecast, as of now: 2"-4" of rain could fall between Thursday into Saturday morning (when the rain is expected to taper off and end). There could be locally higher amounts especially over the S and SE facing slopes of the Catskills, Berkshires, Taconics, Greens and especially the Adirondacks. If these forecast amounts verify there could be some flooding of rivers and streams. While no FLOOD WATCH has yet been issued by the NWS one could be over the next few days.  Saturday afternoon into Sunday should feature a respite from the wet weather but come Monday and Tuesday more rain or showers are forecast to return. Depending on the track and interaction of the "coastal" low will ultimately determine where the heaviest of the rain will fall.