Low Level WInd Shear Probabilities

We are evaluating two ensemble forecasts of low level wind shear (LLWS). LLWS is one of the hazards that are forecast as part of our turbulence package.  Both the AFWA and SREF have been post-processed to produce a probabilistic forecast of low level (0-2000 feet AGL) greater than 30kts.

09Z SREF run on 19 Feb, LLWS probabilities between 21Z Feb 19 - 09Z Feb 21.

12Z AFWA run on 19 Feb, LLWS probabilities between 21Z Feb 19 - 09Z Feb 21.

Experimental Mountain Obsuration Guidance

One of the products that is being tested in the winter experiment is a probabilistic mountain obscuration product based on the Short Range Ensemble Forecasting System (SREF).  The SIERRA G-AIRMET package includes a forecast IFR conditions due to ceiling and visibility, as well as a forecast of extensive mountain obscuration.  The image below depicts
the mountainous regions for which obscuration forecasts are produced.  As you would expect the the major mountain ranges in the US are depicted, but forecasts are also produced for smaller ranges like the Black Hills and the Ozarks.

Image depicting regions of mountains

Automated guidance based on the SREF was developed, using a 40km horizontal grid.  A USGS Digital Elevation Model (DEM) with a 30-second resolution (~0.9km) was used to defined mountains on this 40km grid.  A grid box is mountainous if the elevation within the box varies by at least 1000 feet.  Mountain obscuration is determined for each SREF member.  If a forecast ceiling for a grid box is lower than the highest elevation from the DEM, that point is defined as obscured for that particular member.  Probabilities are then constructed from the percent of members that indicated obscuration at that grid point.

Below is a loop of the mountain obscuration forecast from the 09Z SREF run issued today.  Probabilities are shaded starting at 40%.  Also shown are experimental mountain obscuration G-AIRMETs issued in today's Winter Experiment.

Probability of Mountain Obscuration from the 09Z SREF forecast on 13 Feb.  Overlaid are the experimental mountain obscuration G-AIRMETs issued by the testbed forecaster.

Icing prediction using Mixing Ratios

Several of the products that we are looking at for this experiment use mixing ratios to predict regions of icing.  Both the AFWA and SREF ensembles use mixing ratio from each of the members to predict where icing will occur.

The AFWA takes a worst-case approach for explicit prediction of super-cooled liquid.  Shown below is the ensemble maximum SLD mixing ratio (shaded) with the lowest freezing level from the ensemble contoured over the top.  PIREPs for lgt-mod icing or greater have been overlaid on this forecast.'

11 Feb 00Z AFWA forecast of worst case of SLD mixing ratio and lowest freezing level height.  PIREPs are for lgt-mod icing, or greater.

Post-processing on the SREF takes a slightly different approach, looking at cloud water mixing ratio between 0 and -25 C in each member, and produces a probability that icing will occur.  PIREPs of lgt-mod icing or greater are also shown in this loop.

11 Feb 09Z SREF probability of icing.  PIREPs are for lgt-mod icing, or greater.

IFR Verification

As a follow-up to yesterday's post concerning probabilistic flight rule prediction, here are some quick verification images.  For the C&V prediction, the forecast period is 21Z - 09Z.

Each image contains the METAR observations of flight category overlaid on the gridded data set.  The first gridded data set is an analysis of observed ceiling and visibility conditions, the National Ceiling and Visibility Analysis (NCVA).  Hourly data is show along with the METARs that went into this analysis.

NCVA Field and METAR flight category for 21Z 11 Feb - 09Z 12 Feb.

The primary forecast concern in this area was the lifting of the ceilings, bringing terminals out of IFR conditions.  The dataset shown below is the GFS-LAMP IFR conditions.  Regions shaded in green are where the 18Z GFS-LAMP run from 11 Feb is predicting IFR conditions.  It clears the IFR conditions fairly well in PA/NJ/NY/CT, but keeps DL/MD/VA in IFR conditions too long.

11 Feb 18Z GFS-LAMP forecast valid same period as NCVA.

Additionally, the SREF and AFWA probability of flight category as highlighted in yesterday's post is show below.  The SREF is the 09Z run from Feb 11, and the AFWA is the 00Z run from the same day.  Only the MVFR and IFR conditions are shown in each of these products. The SREF forecast does not seem to discriminate well between MVFR and IFR conditions (all or nothing) and clears the IFR conditions along the northeast coast fairly well.  The AFWA forecast tends to clear the IFR conditions to quickly along the northeast coast.

Another feature of both of these forecast is the forecast areas of IFR around the Great Lakes region that did not verify.  MVFR conditions were observed in MI and NWRN OH.  In the SREF this area is especially overdone and we are again seeing the issues that the SREF has discriminating between MFVR and IFR conditions.  The are in the AFWA is also overdone, with IFR conditions forecast in PA where only MVFR was observed.

11 Feb 09Z SREF forecast valid same period as NCVA.

11 Feb 00Z AFWA forecast valid same period as NCVA.

Ensemble Prediction of Flight Rules

One of the things being explored in the Winter Weather Experiment is the use of mesoscale and high-resolution ensembles for prediction of ceiling and visibility.  The two primary ensembles being explored for this purpose are the Short Range Ensemble Forecasting (SREF) System and the Air Force Weather Agency (AFWA) mesoscale ensemble.

The SREF is composed of 21 ensemble members with post-processed output on a ~16km horizontal resolution, while the AFWA ensemble has 10 members run at a ~4km horizontal resolution.  Each individual member is obtained by the Aviation Weather Center where we perform additional analysis that is not done in the basic ensemble post-processing.

Each of the individual member solutions contains a prediction of ceiling and visibility, for which ensemble statistics are computed.  Additionally, the flight rule category is computed for each ensemble member, from which the probability of the flight category can be determined.

These probabilities for the time period between 21Z Feb 11 - 09Z Feb 12 are shown for the AFWA and the SREF below.  Note that these probabilities are not calibrated and weight each of the ensemble members equally.  Both the weighting of the members and the calibration will need to be explored in order to improve the reliability of these forecasts (see: Reliability and Resolution). 

Probability of Flight Rules as predicted by the 00Z AFWA from 11 Feb 2013.  Valid 21Z 11 Feb - 09Z Feb 12.  MVFR, IFR, and LIFR conditions are shaded in blue, red, and yellow, respectively, shaded starting at 40%.

Probability of Flight Rules as predicted by the 03Z SREF from 11 Feb 2013.  Valid 21Z 11 Feb - 09Z Feb 12.  MVFR, IFR, and LIFR conditions are shaded in blue, red, and yellow, respectively, shaded starting at 40%.